Depends on the gearing of the BB and the quality of the parts like the bearings - and the tyres and pressures I would have though.
I’ve never ridden one of them but people do ride them around flat central London quite happily. So maybe they aren’t so bad on flat roads. What kind of gearing do they have anyway?
Low gearing, partly to drag the weight but also to prevent fitter riders hammering around too fast.
I've ridden them a few times.
Not too bad a ride if you get one in good order, and as you say, most of central London is relatively flat.
So they are ridable, but I wouldn't fancy tackling steep hills, headwinds, or a long ride on one.
One might say the same about an unpowered ebike.