ahaha so many times. usually either when i was with someone else or when i'd at least met the person at least briefly beforehand, like people i met at a social event or something (although ofc these people aren't total strangers). it's not massively safe and i've definitely put myself in hypothetically unsafe situations doing this before, but i believe that most of the percieved risk is just superstition.
uber is no safer, by the way. drivers aren't at all security checked. in fact, uber drivers are in a better position than most people to commit pickup-related crimes because they literally get told where their targets are. most of uber's "safety" statements on their website focus on features which improve the quality of the ride or the convenience for drivers and passengers, but state very little about the actual legitimacy of the people driving the cars.