You raise valid concerns. All I can say is that, for me, and after 3+ years of using them during all 4 seasons of the year in both my coop and my run, they proved their value over and over. The only results with leaves here was good, rich compost and healthy chickens living with effective deep litter. There really isn’t anything that can’t harbor mites or doesn’t have other drawbacks of some kind. How serious those drawbacks are depends on what each person finds an acceptable trade off for what suits the needs of the chickens.