If anyone has found a way to make the AI spawn attached entities, let me know.
Sneakeye or Zycat should have more insight into this.
The AI is very unpredictable how it chooses unit. I found it is heavily influenced by the amount of HP and the cost/HP ratio. The dps also has some influence, but I am not sure which exact settings are used for that. Once I made a new sniper and gave it a custom weapon with different settings but the same dps as other snipers. Somehow the AI never uses it while it spams other snipers.
Also the AI considers two types of units, capture and support. Units with team_weapon_ext or 0 capture_rate are support units and those are build less frequent due to the setting ai_settings > skirmish_settings/army/min_non_support_units_to_support_units_ratio. I gave tanks 1 capture_rate and locked the capture ability with a modifier and now the AI builds much more of them throughout the entire game.
Utility values for a specific unit can be set in ai_economy/complete > squads/squad_utility/utility; keep per_unit_scale and per_unit_scale_historical at -1. This increases the chance that the AI will build it, although in some circumstances the AI still refuses to build it. (This settings is very useful for player owned upgrades)
The moment it takes control, it will reverse the vehicle into its own base, drive up to the HQ and drive out again, about one sector, then reverse back and so on in an endless loop.
Does one of the entities have a dummy weapon? The Katyusha for example has a custom hardpoint 1 and the AI uses it very well at long range due to that. I use this weapon for other artillery units like the Priest too which improves their behavior massively, so a single dummy hardpoint can have a lot of effect. It might be the case that one of your entities has a weapon which the AI don't know how to use?