When I first joined DRN there were a lot of reported piston failures. With that in mind I decided to replace mine in the off season and I ended up being safe. It happens - has happened and will continue to happen. Ratio? I don't really know, maybe 2 out of 10 with just guessing and that probabily probably go's way up if you keep the piston in it for a real long time. Some have cracked kinda early and some have lasted quite a while. If you do the work yourself it's a $150 project, gives you a chance to clean the power valves and gives you confidence in working on your bike.
The reality of the situation is that the stock KDX 220 pistons are prone to a failure which is more frequent than what normally should be expected from piston life.
Be sure to study up on proper handling of the KIPS actuator arm which is reverse threaded and needs to held with a spoke wrench so you don't weaken the shaft and have another potential headache. That's the only tricky part of it really.