In 2004 I bought a ZD21 in partnership with my sister. Between us and my nephew we kept 5 acres of lawn and about two miles of horse trails mowed. The ZD21 was the first generation Kubota Diesel mower and had a lot of issues. It took a lot of work to maintain it including removing the deck every 50 hours to check the gear box oil level and grease the deck. It also was a low serial number and required overfilling the transmission to the point they offered a special tank modification to fill the while transmission case. I maintained it by the book other than I didn't change the mower deck gear box seals every couple of years like they recommended.
We sold it in 2018 with just under 2,000 hours on it, and bought a ZD1211. We looked at about every zero turn made, but my sister (5'-2, 100 lb. couldn't raise the foot push pedal with 60" decks. Neither could my wife, so all of those were rejected). So far the ZD1211, a third generation Kubota Diesel, is working well. The steering controls are much much better. The ZD21 had a lot of slip, the ZD1211 has none, so it's easier to drive. It's also much easier to maintain. There is no need to remove the deck every 50 hours to do routine maintenance. It is specified to go 200 hours between oil changes - the ZD21 was 100 hours. It has noticeably more power than the ZD21. I'm 78, so it will probably be the last mower I'll buy. But with a lawn this big to mow, if I had to buy another one, I'd buy it again. The biggest drawback to the ZD1211 is it's weight. It's quite a bit heavier than other professional zero turns.