I think one problem overlooked with SETs (and I make them) off the top of my head is the huge change in output Z. This can easily be an 8 to one ratio, or more. As an example, the damping factor changing from 8 to just 1 over a portion of the cycle. So the control the amp has over the driver is constantly changing.

PP can also have this problem, in a different way, though to a lesser extent.

Take care.
Steve