I've bred berghia in the past (not really an accomplishment as it's so easy) and I think people get disheartened or conclude they don't work or have disappeared because we are impatient. Adding a few to the average tank can take a few months before you see much of a dent. The best thing to do is either buy a large number to begin with (which is expensive) or get them breeding before introduction. You only need two to breed them but more is better as you have backups and the plan is to build an army to tackle the problem. It can be done in a second system where you transfer rocks with aips over or through the sump (they are nocturnal so if the sump is dark, the process will speed up.
I think they are neat hut not as simple as everyone makes out. I only gave up breeding them as I ran out of aiptasia but the person I got mine from still breeds them and setup a 10ft tank just to culture aiptasia for them.