I don't think a stand-up electric razor scooter is particularly safe if it can travel at bike speeds. The wheels are too small and will have issues with road surfaces far worse than bikes. The danger would be careless riders pitching themself onto the concrete at bike speeds, and anyone who has done that knows it isn't fun. Better full electric bikes, at least they have the tire size to provide a stable ride.
Can they knock a person down? Then we're talking about the possibility of serious injury or death. It doesn't have to be the initial impact, just what happens after that...