Bicycles in big cities are too inflexible. If all you want is to commute like a sheep from home to work and back to home every day then a bicyle is a great inexpensive and green choice, in every other case it's a problem. Unless of course you are in a small village, where a possible change of plans will still result in a short ride. In London this is not possible. If I was to meet friends spontaneously outside my daily commute area then it would be impossible with a bicycle.
The smallest folding bicycles (e.g. Brompton) are fine, but scooters must either be balanced upright, exposing dirty wheels, or take up a lot of floor space. No thanks!
In the largest cities like London, if my plans changed I'd just leave the bicycle at work, and take public transport to the party, home, and to work the next day (unless my way home passes work, then I can collect the bicycle).
It doesn't matter how slow you pedal, in the American Southeast, just being outside in the summer makes you sweat. A lot of places in this area don't have offices with showers in them. A scooter let me spend time outside without having to end up being a mess.
I enjoyed biking in Austin, but as you can imagine the intense heat a good portion of the year meant a full outfit change and shower after every ride just to start the day.
Also they have less of a learning curve compared to biking on roads, and are easier to maintain, IMO.
Me and my friends accidentally ended up testing that theory in LA. We were going to koreatown from Santa Monica and had about 10 miles left. Half of us decided to use limebikes, another half ended up taking uber. The people on limebikes arrived at about the same time, despite the whole scooter group having to stop every 30-40 minutes and search for another scooter, since in a group of 5 people riding those, there would inevitable be one that is running out of charge in that time interval.
Safety wise, I'd say they are about even. Cars aren't going to value your life either way.