From what I've heard and read, it seems to be ok to use as long as the dosage is right. (I also heard the dosage suggestion is different on BettaFix). It's easy to overdose since we dose by drops/screw top caps, so I would either underdose a little to be on the safe side or simply use a treatment without melafix.
Generally I treat my torn fins with clean water and frequent water changes. They are all healing well, and you can add a more natural substance such as IAL leaves to help stress, which will promote healing. Some people prefer to use AQ salt and warm temperature as well, but there's speculation that salt may cause stinging (not harmful, most likely just uncomfortable).