Capabilities - I feel the key to understanding where and when to apply a unit in any given situation is informed by knowing their full capabilities off by heart; here is a look at what this unit is capable of.
While their titular role in the Tau military structure might lead you to believe that these brave Fire Warriors are more capable in a melee or at the very least have that mental fortitude represented in game, such hopes are in vain as Breachers are no different to others of their species in those particular aspects of gameplay. Weapon Skill 2, Strength 3, Initiative 2 and one attack per model is the equivalent of scraping the bottom of the barrel for awful melee fighters, while a base Leadership 7 (or Leadership 8 with a Shas'ui) is less mediocre and more poor by most other factions' standards. No, where the Breachers really earn their name is with some of the most potent short ranged weapons in the game given their status as a cheap Troops choice - certainly no unit in the game with an equivalent points-per-model ratio out-damages Breachers in a fire-fight to my knowledge - even if this isn't supplemented too well by a truly average Ballistic Skill 3 endemic of Tau soldiers. While they might not be the most accurate unit in theory, the Fire Caste has many technologies gifted to it for the sole purpose of addressing that issue; I am of course referring to Markerlights, though the Hunter Cadre and other various rule sources can also affect their accuracy. The Pulse Blasters exclusively utilized by Breachers are not at all dissimilar to Imperial Conversion Beamers even if they do function in reverse in the sense that they are incredibly powerful shotguns that increase in deadliness relative to enemy proximity. When ten Breachers - a basic, cheap Troops choice - can output a jaw-dropping twenty Strength 6 AP3 shots at 5" that can be further affected by an Ethereal's Storm of Fire, a Cadre Fireblade's Volley Fire or Markerlights, you really begin to understand that the Tau don't mess around. Cover can frighteningly be no defense against them, as will hoping they miss with the Pinpoint effects should you deign to provide complementary Markerlight tokens for their benefit in both cases.
It should be fairly obvious that anything that lacks 2+ armor saves, strong invulnerable or (easily stripped) cover saves, Toughness 8 or AV13 may as well be dead meat against their fully powered attacks but the problem lies in closing with the target. As basic Infantry with no added special rules of note affecting their mobility, Breachers are reliant on Devilfishes and other means of transportation such as the Orca to really fulfill their stated role in an army list. Their Toughness 3 and 4+ armor saves additionally don't lend themselves to advancing up the field unprotected, though their in-built Field Amplifier Relays and synergy with optional Guardian Drones are at least somewhat useful solutions to that particular problem and give them a chance of surviving the otherwise murderous Heavy Flamers and similarly potent weapons. Stock Defensive Grenades and the Supporting Fire army special rule act as counter-measures to significantly dampen the impact of oncoming assaults by removing all charge bonuses from the equation and lessening the attackers' numbers or total wound count to somewhat stave off the threat of melee attackers once the Breachers themselves are fully unloaded into the fray. Still, they can nonetheless be seen as the Tau equivalent of a dedicated assault squad by nature of obliterating foes at close ranges, even if they do it in a manner more in keeping with the species' military tactics. Their usage requires precision as they have a large list of viable targets but rely on safe transportation to maximize their theoretical damage output, a quality that is the primary justification for using Breachers instead of Strike Teams - that and their nature as a close-ranged Tau unit automatically gives them a unique place in the list as far as scoring objectives is concerned.
How to Equip Them - Outfitting a unit is often even more difficult than determining the number that you employ, so having appropriate context for each choice is key; this rounds out the most optimal ways to equip a squad.
As a lot of these options are centered around the Shas'ui (Sergeant), I'll focus on that particular aspect of the squad first; the Shas'ui is a useful addition to a Breacher team even if only because of the Leadership buff for what is generally a faction that breaks morale easily. However, the main draw here is the squad-defining wargear available to the Shas'ui alone, namely being the only source of Drones. The first major upgrade is a mix of Markerlight and Target Lock to provide a highly costly albeit "hidden" and unusual Markerlight source that can fire at a separate target to the rest of the parent unit. Some swear by the token target designation systems in units that aren't traditionally home to them as a means of spreading them around and reducing the chances of all or most of the Markerlights being destroyed in one go, though the cost in this particular case leaves me with a rather sour taste. Elsewhere, the Shas'ui has access to two separate Drones that can be taken in the Gun, Shield, Marker or new Guardian variations; the first increase firepower, the second adds sacrificial wounds, the third confers more Markerlights and the fourth generates a squad-wide invulnerable save. Of these, the most useful by far is the Guardian Drone as it combines with the Breachers Field Amplifier Relays to give them all an impressive 5+ invulnerable save, easily beating out the Shield Drone for an overall durability increase. If you are sold on getting your Markerlights in a contrary manner then there's no doubt that Marker Drones are your next best bet, but otherwise you can probably skip past the Drones for what should be a cheap unit. Keep in mind that unlike Strike Teams, Breachers belong in a Devilfish and thus you won't be getting full use out of the Drones until they pop out near the opponent - if you want sources of Markerlights (and this is especially true of the Shas'ui as he has to Snap Fire that tool on the move) then look elsewhere, even if it is Strike Teams.
A possibly role-changing upgrade for Breachers comes in the form of EMP (Haywire) Grenades, providing them not only with medium Strength shooting but also vehicle-busting melee attacks (and a token shooting attack) that make them terrors for anything that isn't 2+ armored or roughly Toughness 8 and greater. I would wholeheartedly recommend EMP Grenades on Breachers if you have the points spare as the mere threat of them breaking open any vehicle they catch serves to present them as even more of a threat to your opponent that must be dealt with as opposed to your more valuable true heavy hitters. Lastly, the DS8 Tactical Support Turret makes its debut for Breacher Teams but is a rather more situation-dependent choice because the nature of the squad makes it invariably suited to constant movement to get the most out of its Pulse Blasters. The Turret itself is fantastic but much more useful on Strike Teams that are better conditioned to immobility, so unless you plan to use your Breachers as potent short ranged objective holders then the Turret is unnecessary for them. Lastly, the narrative-driven Bonding Knife Ritual is a nice upgrade given the poor Leadership score of Breachers and their proclivity for fleeing, though often if the unit has been depleted to the point that the Bonding Knife Ritual takes effect then the squad won't be massively useful from there on out anyway. It is another "if you have spare points" scenario in this case as the benefit is still useful for what will likely be an Objective Secured squad, but it won't come into play too often and is thus not required for general play.
Best Uses - It is true that each unit in the game ultimately has one or a handful of tasks they are best suited for in a general sense; these are the most efficient uses of this unit based on their capabilities.
Given the incredibly short range and exponentially increasing power based on proximity of their weapons, Breachers are suited to close range fire-fights where they can ideally get the first salvo off in a skirmish and punish opponents that fail to appropriately deal with them before engaging. This places a special emphasis on the oft ignored Devilfish, a dedicated transport that is appropriately durable for its price point and thus serves as the natural delivery system for a squad that is too fragile and slow to close with enemies on its own. Pairing the two up is the most obvious and effective use of Breachers as they aren't a better backfield objective holding unit than Strike Teams who have the wider threat bubble to keep them relevant and contributing to the crucial Tau Shooting phase. Being Objective Secured in a Combined Arms detachment on a squad built around close quarters engagements is a rarity for Tau armies, as is having a legitimate reason to ferry units around with Devilfishes that otherwise simply don't require them. Unit sizes of ten are recommended purely because you want to maximise the hitting power that each individual Devilfish protects; the transport itself is costly and durable enough to break the mold of a typical multiple-small-unit build and lessen the value of that army construction strategy.
The alternative use for Breachers is to keep them on your objectives and in cover - Tau infantry are very fragile in reality so it should almost go without saying that you need to use terrain to your advantage - where they can employ a Tactical Support Turret to play shenanigans with line-of-sight blocking buildings or walls, but generally Strike Teams do this job better in most scenarios. Following on from this, the real reason you want a Devilfish is that Breachers can rove forward and utterly murder almost anything with a 3+ or worse armor save but are incapable of receiving damage and surviving against all but the most token firepower - the transport protects them before they can get into that optimal 5" range. Thankfully, the similarly recent addition of Guardian Drones to the codex provides you with an unusual situation; your basic infantry can survive even Heavy Flamers by virtue of employing squad-wide 5+ invulnerable saves so cover isn't quite as necessary. Lastly, as inexpensive as they are by themselves, they function like any other Tau unit in the sense that they require support to really unlock their full potential (the most obvious being the Devilfish). One Markerlight hit and a lack of available cover saves is enough for a full Breacher Team at their shortest range to murder an entire Space Marine squad or destroy something akin to a Tervigon or Daemon Prince, all assuming average rolls, but with cover and no Markerlights that efficiency drops significantly. In that sense you will get what you put in with Breachers and they are fun to use in a Tau army because they reward deeper thinking and smart implementation into an army list - do you need a fast (again, with a Devilfish) scoring unit that packs a punch, for example - though obviously they are more points-hungry than the static Strike Teams.
Thank you all for reading this article, I hope you enjoyed it! Breacher Teams are a bit of an unusual unit by Tau standards given that they operate by closing with enemies rather than distancing themselves from enemy aggression, something that I feel gives justice to the oft forgotten Devilfish by providing it with an ideal squad to ferry around. If you agree or disagree, feel free to leave a comment below and I will take all of your feedback into consideration. Thanks again!
The strength of your force may be calculated by multiplying its weight by its velocity. Strive always to maximize both and victory shall be yours.
- Commander Puretide, Legendary Tactician of the Dal'yth Sept