They can work as well as the old trick of leading a hose from the bleeder into a fluid filled jar... which you will probably be doing anyway to avoid a mess.

Sometimes air can get sucked in around the threads of the bleeder when you let up.

I have not used the speed bleeders but have heard of the problems, and I have had that problem with the old style "one-man" bleeding hose-in-jar.
You have to just barely loosen the bleeder, and if it has ever been overtightened, or the seat is at all corroded, it can happen.

I prefer gravity bleeding and/or pressure bleeding, or using 2 people.