BTA would not be bad if it get shedded in a tank other than nutrient load. However if one of the more potent anemones get shedded, the nematocysts released in the water will kill all the fishes other than clowns. I have not heard of BTA, Crispa or Malu causing this problem. I read about Haddoni, Gigantea, Magnifica causing tank wipe out.
Personally I have never experience any of my anemone shredded in my aquariums.
I think the risk tends to be exaggerated. Especially for bta’s. Bta’s have a really mild sting. You should take steps to prevent an anemone from getting into a powerhead anyway, but occasionally they do make it past guards. I always run carbon with my anemones and that should help if you were to have an incident. I’ve had haddoni manage to get a portion of their body past a guard in about a 35 gallon tank and everything was fine. Granted it wasn’t the whole anemone that was injured.