You’d have to constantly test. Just because a lines worker tested that the line is dead five seconds ago doesn’t mean some idiot just plugged one of these in.

They can work, you just need to disconnect your house first then use it. It’s also a good way to burn your house down. If something on the same circuit as the generator pulls more current than the wires in the walls are designed to take, because there’s no breaker in the way anymore it can catch fire.