Maximum power output and maximum (assisted) speed are generally legislated. In the UK, an e-bike is up to 250 W and 25 kph. More than that, it would classify as a motorbike and you'd need a license (not particularly onerous). The bike itself is often built differently to accommodate the different power profile.

As a pedal cyclist, I feel that's a reasonably sensible limit as much faster than that you should be more experienced as a cyclist to control the bike and anticipate the conditions.