I've used melafix with multiple gourami species and a group of senegal bichir when they have had ripped fins on more than one occasion. I have never noticed any adverse effects when I have been using it and if fish have died it is always outside treatment after multiple water changes so I have no reason to suspect melafix as the culprit.
I know of shops that use it with their gourami species, bettas, bichirs and snakeheads without a problem.
I have read about melafix and pimafix with people having bad experiences with them but when you consider the amount of those stories and the amount of fishkeepers it is probably about the same complainant rate as adverse effects of other aquarium treatments.
Probably even less because it is a natural product and not regular chemicals such as ich treatments or other fin rot treatments made by the popular manufacturers.
Imo, even if the whole of MFK complained about melafix having adverse effects it would probably still amount to a tiny percentage of complainants compared to the amount of fishkeepers in the world that use it.
The horror stories, some probably started by the competition, will not put me off using it in future.