Um. I unabashedly love that idea.First up, when it comes to 'game breaking' blame the individual player. It's clear that the 'page 5' of 40k is about telling a story. Give your battle a backstory. Build up grudges.
The result of the game should be secondary to the playing.
For instance, I am tempted to add some Guardsmen to my Nercons, to represent mind shackled slaves. But first, I need to beat an IG Player, how else am I meant to get those Scarabs into their cortex? After a string of victories, I can start looking at including more exotic IG units, and bigger Bastions and defences.
They add an extra dimension to the game for players to explore. And if someone chooses to exploit rather than explore, then that's their call. I'll play them, but likely only once.