Even still, it's the kind of chemical that should only be used if there is a problem. Saying it works because you've never had a problem is like saying concrete floats cos you've never seen it sink. I know lots of people who've never had a problem with mites ( me included) who've never treated for them.
Don't mean that as having a go at you, but overuse of a chemical in anyway can have adverse effects if a problem occurs. If one mite survives a treatment, it's stronger against the chemical, times that by 5,6,7 and you;ve got an almost completely resistent mite.
Again, sorry. I just though I'd stress the point. There are many reasons to not use chemicals where possible.
Sorry to hear about all this trouble, I hope you and your animals get through it alright.