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.