What are you comparing a ZTR to to say it has a high center of gravity? My ZTR has a lower center of gravity than any other mower I have. This includes a F series Kubota, a Kubota G3200, a John deer F525, a B7200, and a regular riding mower made by Cub cadet. A ZTR can't compete with a real slope mower rated to something ridiculous like 45 degrees, but with locking front wheel would do better than a regular riding mower.
you might be right on the lower center of gravity for ZTR's 60" or larger, but there are plenty of smaller ones that have both narrow & short wheel base...add the rider weight and sometimes hyper sensitive hydro, and you have a dangerous situation on a wet slope. my only point i guess... but i still don't buy locking ft swivel wheels, too dangerous.