for e-bikes there the pedal assist has to cut out when going over 25 km/h (+10% error, so at a maximum 27.5 km/h). So of course you can speed down a hill at 55 km/h, but it has to be unassisted. The motor cannot be more than 250W nominal power.

You can actually buy e-bikes going up to 45 km/h but they are regulated as scooters and has to be registered (usually free), requires a drivers license and the use of moped helmets and you're usually not allowed to use them in bike lanes. Oh, and you need insurance too. So for most people it's not worth the hassle.