They may be less dangerous than a bike because they can go quite fast, matching the speed of cars when riding on the road, which helps you avoid dangerous situations in which car drivers try to pass you without enough space to do so. Good bicycle infrastructure can minimize this, though it doesn't seem like Denver has it.
- the usual pedal-assisted ebike which is classified as a bicycle, pedal assistance up to 25 kmh
- bikes with pedal assistance up to 45 kmh, these need to be registered and need a special licence plate, and you need a driving licence to ride them. Bonus is (compared to a scooter) that if you turn the motor off then they are treated as normal bicycles, so you are allowed on all the bike paths where regular bikes are.
These latter ones are actually fast enough to keep up with city traffic easily, even though in Zurich there is a lot of bike infrastructure, so you wouldn't necessarily need them. I think they are the best of both worlds, bicycles and mopeds.
The 50cc/45 kmh or 4kw/45 kmh for electric ones defines a class of vehicles (mopeds L1, L1e-b if two wheels) that have a simpler registration and need a simpler driving license.
Electric powered bikes exist (Powered cycles, L1e-a) with 1 kW/25 kmh, while electric assisted bycicles and scooters[1] are "out of scope".
It's complicated:
https://en.wikipedia.org/wiki/Vehicle_category
[1] the kind of electric sccoters with tiny wheels and no seat, "scooter", at least here in Italy, is commonly used to call anything with smaller wheels than a motorbike, it is used also for (powerful) motorbikes of class L3.