>>They must obey standard speed limits so 50 km/h in cities or 90 outside (good luck with both of them) or whatever the limit is.

There was a court case about this actually, lol. A cyclist was caught by a police car going above the posted speed limit(I think it was 30km/h where he was), and he refused to accept the ticket. The case went to court and he argued that since the law doesn't require bicycles to be fitted with any instrument to measure speed, he is not required to know the exact speed he's travelling at. And weirdly....the court agreed and the ticket was cancelled.

why is that weird? seems like a quite reasonable argument

Because in almost all other situations of life you are required to obey the law even if you aren't aware of it. For a different example - cyclists aren't legally required to pass a driving test, yet are required to be familiar with, and obey all rules of the road. You couldn't argue in court that as a cyclist you don't have to stop on red because the law doesn't require you to pass a theory test before cycling. I find the situation similar - you should make sure not to go over the speed limit, even if the law doesn't require you to carry a speed measuring device with you.

fair points

https://eur-lex.europa.eu/legal-content/en/ALL/?uri=CELEX%3A... specifies that "Light two-wheel powered vehicle" can only go up to 45km/h.

Edit: so, yes, no speed limit for bikes, but speed limit for ebikes.

Yes, the difference is the "powered" part in the cited document. It's not terribly clear because they don't explicitly exclude acoustic bikes but they also never include acoustic bikes in any category that is to be legislated (article 4).

Can't be an EU law, few things are outside of the very specific scope of product feature requirements. Germany (part of the EU) has or exactly reverse: explicitly posted speed limits apply to bikes just like they do to powered vehicles, but the law that defines the implicit speed limits (50 inside cities, 100 outside) is written with wording that explicitly restricts its applicability to powered vehicles (that's how the superset of motorcycles, cars and trucks is usually written). This certainly wasn't the intention of that law, but it has been tested in court. When you use that loophole, "they" can't do anything about it other than invoking the catchall law of "don't do unreasonably irresponsible shit in traffic" (which certainly they can)

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.

Something like the eRockit is a better idea.