I'll elaborate. I have a Kubota ZT. Like most ZT's all the weight is in the back, and where you sit doesn't cancel that. Sure, you're forward of the engine, but still near the rear third of the mower.
I have hills and it really doesn't take much slope to make me nervous. You have these hairtrigger controls and the ZT's can really accelerate, and I definitely pull uphill wheelies without meaning to, and have had the whole thing attempt to turn over on me if not for the fact that I was trying to get out of a ditch I was mowing such that the rear of the mower was halted by one side of the ditch.
Very. Sensitive. To. Hills. More dangerous than my tractor. Would never dream of asking my spouse to drive it.
Maybe some ZT's are better, or maybe people's idea of "hills" vary. Some people also use weights on the front of the mower. Even without the rollover factor there are other issues with ZT's on hills, e.g. those hydraulics are your brakes. No hydraulics, no brakes!
Anyway, careful, and if you have hills, take it seriously and test what you're buying on some hill. For my mower and yard at least it is no joke. Like the tractor, consider mowing in reverse to go uphill if rollovers are a concern. Though I have horrible luck with my tires and traction when I try to back up, which has nearly left me stuck from time to time.
On the bright side, out on the flat straightaway, I'm flying and it's a hoot.