Where should I put a Siege Breaker?
>HSS
Good guns that benefit from his Cognis...but frail and can't melee, so the SB can't use his hammer
>Command & Control
Their purpose is to just stand there trying to farm one reaction... meaning they can't fight at all
>Veteran Tacticals
Despite being hardy, they can't really fight, and most of their shooting is single-user combis anyways. Or storm bolters
>Cataphractii
Same shooting (combis). They can fight, but they won't be using that cognis much

Where should I put the Siege Breaker?
Worst of all, he unlocks NONE of the previous units: he gives you up to two Rapiers only and two Arquitors-Vindicators only.

It is very good that Veterans can now bring heavy weapons other than missiles. Even though missiles themselves are alright :D
But how to kit the rest of the unit?