I also use a Rollei with prism. You can actually orient the camera like binoculars if you want better balance though I have no trouble with the balance holding it normally. The problem with hand holding a Rollei with or without prism is that it slows down your film advancing. You have to grip the camera one way to take a picture and re grip it another way to advance the film.
On a tripod it is much quicker.
I also use a Pentax 67 with prism. The problem I have with that in portraits is that the slap of the mirror causes people to blink right when the shutter opens. I have lost nearly whole rolls to some who blinked on every shot. The solution to that is the mirror lock up but then you have to have the camera on a tripod.