This is only true insofar as the levies aren't so unhappy with our leadership that he can't tell them "Y'know what? Let's just kill them all" and have them follow him.
He won't need much diplomacy for that, if he's been leading our armies to victory after victory - and geniuses are unstable, which means he might just to that.
Geniuses aren't inherently unstable.
Only 1.5 out of our 4 Geniuses have been unstable.
Yeyena was full on cray cray (aka touched by the Warp) but she was also essentially harmless. She didn't fly into uncontrollable rages or kill people on accident or anything.
Our astrologer Mystic Genius (who I forgot the name of) wasn't crazy, she was just mute and really, really smart.
Our Genius Admin (Yshuin sp?) didn't start out crazy, but we made him crack because we decided to start a bloody offensive war while having a super empathetic King.
We never even saw our second Genius Mystic, but from the fact that he had a shit ton of followers leads me to believe that he was stable enough to at least fit into society.
Edit:
We also had angry blacksmith man (Genius Artisan) who would sometimes verbally lash out at people while he worked, and was in general very passionate about his work, but I wouldn't call having a temper unstable
Basically, so long as we don't try to make the Genius (who is guaranteed
at least semi-stable) crack like an egg, we will be fine.