Mapman, A good SET amp is at least as reliable as a push-pull tube amp, and in many cases a SET is about as reliable and trouble-free amp there is. For instance, my SE amps use a 46 output tube and a Western Electric 403b driver tube. Neither tube has any adjustments and each tube should last for many, many years with zero maintenance. The only control is a power switch. How easy and trouble-free is that?
A SET amp is possibly more "variable" due to speaker compatability but I can't think of any other reason why someone would say this. It is certainly not due to "how a particular design distorts."
SET amps vary all across the map in terms of circuit complexity and parts quality and this results in a wide variance in price. This is also true, however, for push-pull tube amps and solid-state amps.
The real reason for choosing a SET amp should be its sound quality, and with the right speaker the sound quality of a single-ended amp can be very special.
