Any medication is harmful if you use it incorrectly.
BettaFix is not inherently harmful, there are lots of horror stories about MELAFIX though, and that one definitely should not be used with bettas.
BettaFix is specifically formulated for bettas. Knowing the risk, you should only use it in amounts prescribed and in small tanks.
I use BettaFix, and my fish react very well to it. It definitely depends on the fish, although the risk of coating the labyrinth is low to none with BettaFix, most fish have an immediate stress reaction to any medication.
It wouldn't be on the market if it was responsible for copious amounts of betta deaths, as projected by some people. Of course, your judgement is best, and there are countless alternatives. If you feel there is a risk, you are correct in not wanting to take it.