I put about 20 of these guys into my 50 gallon two years ago. They seemed to be doing okay at first but eventually disappeared, and it wasn't for lack of aiptasia because I had hundreds if not thousands.
I then bought 3 decent sized peppermint shrimp (the true ones not the camel shrimp)...those bad boys took care of business and my tank was clear in less than two months and stayed that way.
I really think peppermints are better because they are:
1) cheaper
2) hardier, they eat the same frozen stuff as the fish and don't just die when the aiptasia are gone like the berghia
3) they don't pick at any corals or other stuff that I ever saw, only the aiptasia, so in that respect same as the berghia
4) more interesting than the berghias
Just my two cents to keep people from spending a ton of money on these nudi's that are hard to work with IMO.