There are a few diferent types of flatworms, I wouldn't say any are really good.
The worst I can think of are Acro Eating flatworms, as the name would suggest they eat Acropora, and there is no full tank cure, the treatment is almost as devastating as the bug and they are about enough to drive you out of the hobby. You get these usually by trading or buying acros from somebody with an infected tank.
The other 2 kinds I can think of don't eat coral, those are red and white flatworms, you get these in the same way, getting a rock or coral that is infected, but they are not nearly as bad. These can be wiped out by salifert FWE with one catch, they are poisonous and if you have too many, too much of the toxin can build up in your tank and wipe it out. It is always best to deal with these when you first notice them to avoid this problem.
There are also Nudibranchs, but that is a whole different thread, they are not flatworms, but some people call them that.