And my experienced are totally different. Out of the 100 hives i did last spring, i think i lost 3 queens, and 2 of them were due to formic to soon after requeening. My honey yields did not suffer.
However i followed the directions, and i made sure there were atleast 5 frames of bees per box. This meant that some hives needed to get combined, and some hives were combined with a queen excluder, and ready to split at the end of the treatment.
All hives treated were bursting at the seams - except for those three colonies - and ready to be split at the end of the treatment.
There was one yard that did not get done, and that yard performed poorly the whole year.
I have used formic (MA2) more than once and it is a product i choose to use to control mites.