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.
An Ironstrider Ballistarius is well equipped to deal with light threats to your force and has numerous synergistic uses, though it is difficult to escape the mindset of it being a more powerful Sentinel variant. Ballistic Skill 4 and either a twin-linked Cognis Autocannon or twin-linked Cognis Lascannon make for a reliable light vehicle hunter that can easily pop the odd wound or hull point off of the appropriate large target where necessary or gun down a model of lesser stature. The lack of additional weapon options narrow the diversity of roles a Ballistarius can fill as it isn't capable of dealing that much damage in a given turn, but the main advantage it does have is that it can be easily positioned to strike at vulnerable points in a battle line such as exposed weaker armor values on vehicles. This is because it too is a Dunestrider model and thus moves 9" in the Movement Phase, while Crusader is beneficial for getting it out of trouble or getting off a Sweeping Advance in the unlikely event that it manages to win a combat. Add in the fact that it applies Precision Shots to its ranged attacks and it quickly earmarks itself as a potentially lethal assassin, especially if the Lascannon option is taken.
The Doctrina Imperatives can further boost its accuracy or melee capabilities on the fly but ultimately the Ballistarius tends not to rely on them as much as other Skitarii units tend to, especially given that a paltry two Strength 5 attacks at Initiative 3 backed by Weapon Skill 3 will rarely impress regardless of buffs to that latter characteristic value. Ballistarii generally want to avoid combat save for trying to tie up a unit in combat, getting a few extra hits on to a depleted squad or slug a hull point off of a nearby vehicle - the Cognis rule assists in keeping out via Overwatch and also allows it to double as a decent deterrent for both living and mechanical aircraft. AV11 on all sides is fairly decent for a light walker of this price point, though two hull points and the Open-Topped classification do a Ballistarius no favors; the lack of an in-built cover save akin to the Dunestrider makes the Ballistarius a prime target for light to medium anti-tank weaponry. Of course, having access only to long ranged weapons allows you to keep Ballistarii out of unfavorable fire-fights fairly easily and this is especially the case if you take full advantage of that aforementioned 9" move. Broad Spectrum Data-Tethers that boost friendly Skitarii Leadership values upon activating Doctrina Imperatives and Searchlights round out the Ballistarius' equipment, giving it some added use as part of a moving or static gun-line force.
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.
Seeing as Ironstrider Ballistarii only have two wargear selections to choose from, boiling down to which gun you want it to use, I'll keep this brief. The significant points differential between the Autocannon and the paid-for Lascannon generally keep the former weapon in favor as far as I'm concerned, especially when two Strength 7 AP4 shots will often be as good or better than a single Strength 9 AP2 shot against quite a few targets. However, the high accuracy offered by Ballistic Skill 4 and twin-linking the weapon in question also gives considerable justification to use the Lascannon instead - that shot can inflict Instant Death on Toughness 4 foes, potentially cause an Explodes result against a vehicle, actually damage AV14 targets and crucially ignore 2+ armor saves. Couple that with Precision Shots and it quickly becomes a huge threat to opponents baby-sitting their characters with bodyguards, raising the overall value of the Ballistarius as a viable target in the eyes of your opponent and thus drawing attention to itself and away from your more expensive friendly forces. It's legitimately a tough question that I tend to answer with points and the idea that the Ballistarius is a cheap scout and distraction unit rather than something to invest heavily in, but both weapons have their merits - I prefer the Autocannon but I would hesitate to say it is necessarily the better option.
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.
Whether roving as a single Balistarius or as a pack of Balistarii, these unusual walkers are your light ranged suppressors to the glass melee bullies in the Dragoons; though their preferred phase of warfare is different, their role is intrinsically shared. You don't want to invest too heavily into them as their return-on-investment will generally be low in the sense that they can't stand up to too much punishment and aren't equipped to out-damage most enemies when trading blows. However, they are impressive in their swiftness and the fact that they have 48" ranges on their guns helps to keep them safe and out of reach where they can feasibly generate victory points for you. They are a perfect tool for gaining Linebreaker, Hold the Line or specific Secure Objective cards at a moments' notice as they can't be affected by Leadership and are thus fully self-reliant, deftly complementing their speed.
Units like these are useful often because they either generate victory points tied to board control or eat up your opponents' resources on a circumstantial basis, but when they are as efficient as Ballistarii are then including one or two in an army list is nearly always advisable. Front AV11 and the changes to using grenades in close combat give them a lot of leeway to charge units of varying sizes and tie them up, even if the Ballistarius itself is unlikely to ever actually destroy anything in combat. While you probably should keep the Ballistarius away from combat and close ranged shooting, it's just one of many uses of the walker. I tend to recommend against squadrons for Ballistarii more-so than the Dragoons as these don't really require numbers to do their job effectively; a squadron of Dragoons can be difficult to shift and deliver strategically-placed hammer blows to an enemy battle-line - performing a role that no other Skitarii unit can emulate. On the flip side, a similarly-styled Ballistarii unit will act as a more durable but less efficient equivalent to your Onager Dunecrawlers which are the true ranged heavy hitters of a Skitarii force - if you want a squadron of walkers to deliver death from afar, the other Heavy Support choice is where it is at and not the scouting unit.
Thank you all for reading this article, I hope you enjoyed it! Ironstrider Ballistarii are very much the ranged equivalent of Sydonian Dragoons in more ways than one, necessitating finesse with their strikes to really maximize their effective usage. I'm eager to read your thoughts on the fondly named "chicken walkers" and how they have performed in your games as part of a Skitarii force. Thanks again!
The Omnissiah directs out footsteps along the path of knowledge.
- Techpriest Yaffel of the Cult Mechanicus