They'll go straight up a hill (not too steep) no problem, with control, but coming down a hill isn't good. I have an area where I have to go partway down a hill then turn to continue mowing. Often I can't, at least not without skidding the inside tire trying to get it to turn. Better on the lawn to go all the way down and turn around, go back up and then turn. I can drive along the side of the slope as long as I don't try too hard to then go up, because it'll skid that inside (high) tire then, too.
Mine is an EverRide Warrior. Great, super heavy duty machine and my first zero radius mower, but the negatives are that I eat much more dust, the ride is rougher, and they're harder to control than a sit on top lawn tractor. But they are fast, super maneuverable with turns, etc. and that's the one I use at my place.
Hope that makes sense. Try one out before you buy. They take a little while to get the hang of, controlling and steering, but try to see whether it'll work for your property.