Power to make people insane. Hmmm... a pretty useless power once you think about it. Who'd want to make someone glitchy strong insane? It's like adding fire to gunpowder. Guarantied death.
Flandre doesn't mind losing her sanity. Sane people want to keep their sanity so becoming insane isn't their norm and thus become useless. Flan just becomes twice as destructive.