My grandmother used to have those edison sockets in the bare bulb fixtures in the basement.
They each ran a fluorescent strip light. Safe? Yes and no. It was way under the limit as far as wattage/amperage goes, so it was not being overloaded. It was pretty much the same as a 40 watt bulb in the fixture.
BUT! The cord for one of the fluorescent lights was an old grounded 3 prong cord. It was plugged into a small brown extension cord with the grounding prong bent outward. Then plugged into the edison socket.
Also plugged into that small brown cord, which wasn't intensionally done because the lamp socket was around the corner, was a cord running to a workstation and another to a fan. The little outlet socket was burnt into the lampholder from heavy load probably peaking 1,000w or more at times! The breaker/or fuse never blew/tripped either because it never exceeded the household wiring capacity. Usually 1,875w 15a.
They are safe if use right. But you have the people that "don't know anything" and think "if it fits, it works!" Best bet is DON'T use them. In fact, they should stop selling them because of the high risks of accidental misuse.
If you have a fixture that you want to plug something into, its easy to tap off of that or a nearby source with the correct gauge wire, connections and box, to install an outlet that will give you all the power you need.
Or, remove the light, and put an outlet right into the box. They are interchangeable. Just make sure that the wiring gauge is correct inside the box that matches the fuse/breaker.