I think the reason it is not in common usage is that formic is a strong acid. It requires inhibitors to be added when used for vehicle cleaning, for instance (or it repidly removes zinc coatings on nuts, bolts and anything else). So it really requires protective clothing and a sensible/careful operator experienced with aggressive chemicals.
I am not sure whether it should have an 'irritant' chemical warning label or a 'corrosive' one at the concentration used. Pure acid will have a 'corrosive' label and, as such , certainly cannot be transported by all and sundry - eg not by post. I am thinking 70%, the usual concentration used, will be the same.
It is a volatile liquid and can, if not controlled carefully, either be ineffective or may cause abscondment. From that point of view it needs greater attention to temperature and dose than thymol.
It works just fine. But I would not make a recommendation to all and sundry, especially those with no chemical experience, only a couple of hives, or a nervous disposition.
Bigger drop can mean greater efficacy, so a larger proportion of the mites removed, so the size of the colony is immaterial and it gets to the mites in cells so, arguably, there are fewer damaged bees emerging when treating with formic tha thymol preparations.
It can be a cheaper method too, after the applicators have been purchased.
Regards, RAB