The motors in these moped/scooter style are often direct drive. I have ridden a few at Shanghai cycle show,a genuine 250 watts barely moves the weight of the bike plus my 105 KGs. 350 watts and it starts to become useable if you weigh less than 70 KGs,any more and it won't drag up a hill,I managed to get 20 mph out of one but it took for ever to get there.
Most of the cheap Chinese are 800 watt and will move a little chinese person around at 35 mph,you often see them loaded with Mum,Gran and 2 kids,they move ok but only on the flat( most Chinese cities are flat being built on old flood plains).
So to be useable with our European weight you really do need 800 watts,obviously this uses batteries up quick so 20 Ah is really the minimum.
But it is almost impossible for the police to tell the difference between 250 watts and 800 watts so the importers/sellers feel quite comfortable in telling you that their bike is 100 per cent legal.
But the Achilles heel of illegality is weight,most weigh 60-70 kilos (max legal 40 kilos) and it's not classed as a tandem,if you buy one you become adverse to letting anyone weigh one because innocence of the true weight is probably your only defence and the seller told you it was legal.
I have a couple of 250 watt Chinese cheapies at our warehouse,they are sort of bikes and sort of mopeds,fully stripped down they weigh 37 kilos but anyone over 60 kilos would be very disappointed with the performance.
No rubbish,no ulterior selling motives,this is honestly what I know about these bikes. If you want 30 mph plus performance it has to be illegal as a bike and let no one persuade you otherwise,but for some the illegality does not matter.
KudosDave