The advantage of a ZT mower is maneuverability ("turnability"). On a flat out rectangular open area, they probably are not much faster than a tractor mounted mower.
We bought a ZT a few months ago. We love it! We even enlarged our yard

Because of the obstructions in our yard (chicken pen, hills, driveway, patio, fish pond, flower beds, etc.), there was so much time wasted turning around with the tractor ( 27hp
4wd diesel w/ 5' midmount mower).
I don't worry about the ZT tipping on the hillsides, BUT it will lose control going across a slope and suddenly head downhill and there's not a thing you can do to correct it. They are good at going up hills, but may have problems going downhill or turning around on a hill. I'm sure it depends on how the individual model is engineered. There is apparently a great deal of difference in how various models handle hills.
Also, a ZT can be a very rough ride if the ground is not smooth.