For mites to develop resistance to either formic acid or oxalic acid would require massive changes to the actual mite, not just a few changes in DNA. So it is far off, if ever, that we need worry about mite resistance to those.
Amitraz, the active ingredient of Apivar Strips, has been used in it's raw form by commercial beekeepers, year in and year out, without resistance developing (that we know of). There have been rumours of resistance but i have not seen those confirmed. So at least for now, a person is pretty safe with Apivar strips, long as they are properly positioned and left in the hive long enough. Kuro i'd be interested to see a link about cattle ticks getting resistant to amitraz, if you have one.
Thymol in the form of Apiguard is very effective, IF weather conditions are right and the colony set up properly. But can be pretty useless if either of those conditions are not met. In my view this product falls into the experienced beekeeper only catagory. But the other thymol product, thymovar, is very unreliable, falls into the don't waste your money on it catagory.
Some of the other mite remedies that are for sale are pretty useless, in fact in my view worse than useless. Because using them can give the beekeeper a false sense of security and they take no further action, and lose their bees to mites. In my view some of these treatments should not even be sold, thymovar and hopguard being two examples.