I have seen Unholds (for sure) and Fallen Heroes (not entirely sure) use attacks that hit 3 tiles and not attacking the allies standing in some of those tiles (while causing the damage to my units).
Maybe that’s a bit unfair and unrealistic?
My suggestion: enable friendly fire for AI opponents (Unholds and Fallen heroes at least). The former are huge and stupid, the latter are rotten-brained and should not care about their undead brethren.
I think it’s fair, it’s fun and easy to implement.
I strongly believe the game will only win if we not only get moments of regret when our brethren fall, but also have rare cheers when enemies hit one another.