They change more than once per invasion.
That doesn't mean that none of them ever return, though. It just means that this is the third invasion with daimons in it, and if I remember correctly more than 4 GMs played them. I can't quite remember how many, though. I've pretty much always been in contact with the daimons, so any changes were noticeable. Also consider that there were more than 1 on at least some occasions.
Players get bored after a while, after all, why wouldn't GMs? A lot of things can be quite frustrating for them too.
You mean like people whining about inconsistencies when they are giving up their free time to try and deliver an engaging story line? Yeah I can see how the constant pressure of unfulfilled player expectations might end up being quite frustrating.
