Well, to answer the OP's question, the line is drawn (in the US) for MUPs at Class 1 or 2. 20 mph max. Some municipalities have lower limits; 15 mph is common.
You can go and look up your state's laws just
like I did for more clarification.
How we FEEL about those laws and whether/how well they're enforced is something different. Many pages of posts in many threads have been typed about it already. We have guys that want to ride 50 mph offroad electric motorcycles with vestigal pedals and then we have guys who think it should be pedal assist only, and only up to 15 mph. Then, everything between.