I've only played a couple games so I know my experience could be faulty and I'm definitely comparing Mansions of Madness to Arkham Horror where loss of sanity and loss on health are roughly equivalent.
That said, as the keeper player I haven't found causing a player to go Insane to be very effective. The ability to play a trauma card as a keeper action doesn't seem very strong. By the time you've driven someone insane, it is late in the game and Threat is at a premium. I haven't been able to get enough Insanity Trauma cards to take advantage of the Insane person.
How can I better take advantage of an Insane Investigator?