Same size of hazard that we have now with inexperienced and fast people on conventional bikes. If they tangle with the rest of street traffic they come out second best.
BTW on my "Mud Adder" I have five levels of pedal assist and a nine gear cluster. Using PA 4 and gear eight on the flat gives me a nice 28 km/h when pedaling at 84 RPM on the rail-trail and a bit faster on pavement . Not to mention gear nine and PA 5.
What surprises me is the fact that many of the slower riders don't have mirrors to check what's coming up from behind and quite often seem to be day dreaming while taking up however much of the trail as they desire.