I think throttle-only would make them illegal in most EU countries (or, more specifically, they wouldn't be in the same category as bikes anymore); but also, is there really anything implicit in a mid-drive engine that prevents throttle-only?

Interesting point about the chain. What causes more strain, specifically? Essentially, if a certain power is required to go at a certain speed, what makes it different if it's muscle only, or mixed muscle/engine?