They are completely safe. There are two kinds. One is what you would call an adapter. This is a plate that can be anywhere from 1/2" to 3". The plate is bolted to the hub and the wheel is then bolted to the plate. The other kind is a thin plate (1/8"-1/4") that sits between the wheel and the hub, but the wheel is still bolted onto the hub itself. If you are using the thin spacer, you must make sure that you have 7-8 full turns of the lug nuts to be safe. 6 is ok, but you should check them every now and then. If you only have 4-5 turns, you need to get an E.T. style lug nut. These are made with an extra shank to reach down inside the spacer and get more threads. If using the adapter style spacers, use some lok-tite on the studs on the hub because you probably won't want to take the wheels off very often to check for tightness of the lug nuts holding the adapter on. Make sure to get them good and snug and make sure that the stud on the hub doesn't stick out past the adapter because it won't allow the wheel to sit flush on the adapter if it does.
These adapter are safe if properly used, but can be very dangerous if installed incorrectly.