The only real diff with the French laws is that throttle only is banned in France and legal (currently, more or less) in the UK.
There's only really three[1] styles of hub motor out there.
- Direct drive. No internal freewheel or gearing. Significant motor-off drag as it works like a generator. eg Crystalite.
- Geared. A planetary gearbox has the planet gears mounted on a freewheel. Motor-off, the planetary carrier revolves around the motor sun gear. There's additional drag due to the gears and freewheel carrier revolving. eg Bafang, Cute, BMC, Mac, Puma, Ezee. The Cute has a two stage gear where the planets are two piece and different sizes. One contacts the sun and the other contacts the ring gear. This gives them a 9:1 ish ratio instead of 4.5:1 ish which then allows a faster smaller lighter motor.
- Tongxin. Like the geared except that the planet gears are actually smooth and are a loose fit so they just fall away from the ring surface. Has the lowest drag because all that's left is a bit of grease drag between the planets and ring. However, Tongxin hubs are expensive, hard to get hold of, have some reliability issues and are low power.
As well as the design issues above, hub motors have more bearings than a conventional hub and they tend to be sealed bearings with higher drag. Typical bicycle bearings when well adjusted and lightly greased/oiled are very very low drag. The end result of all this is that the typical Bafang/Cute has noticeable drag and a spin down time of perhaps 20s compared with minutes for a conventional bicycle wheel. This drag does seem to be vary between individual hubs of the same type and does perhaps improve with use as everything wears in. It's also very deceptive as it's virtually un-noticeable at walking pace but feels like under-inflated tyres at 15mph and above.
In practice, if you use a little power all this is moot. It's only really an issue when you run the battery down and have to get home! The Bafang motors are very widely used, and are a safe choice. They're only possible downside is that people on ES trying to vastly over-volt and over-amp them stripped gears with the abuse on a batch of motors 2 years ago.
[1] The non-hub Panasonic system has a simple and more effective freewheel that does a better job than hub motors of isolating the drive from the motor. Then there's friction drives that are pure DIY at the moment, but have zero drag because the motor is completely removed from the bicycle systems when powered off.
