As with many other things, it depends on how you ride. They do have some risks that bikes don't have, (the worst one being that if they fail while you're riding you face plant), but by the time I bought my last one several years ago, they were very very reliable. I would consider them to be much more safe than scooters because the big wheels are much better at taking potholes, plus your hands are free and it's easy to jump out if needed.

That said, the new ones go way too fast (My comfortable speed was about 20 to 25 mph on the streets, a lot of the new ones go like 90). However you don't have to buy the top end ones, I actually preferred the little ones because they were more practical, and you don't want to ride them at close to max speed anyway, just like with cars.