My anecdotal evidence: I have NEVER had a case of scaly leg since I started using DE in my hens' coop and nest boxes three and a half years ago.
The birds I had in the past had me struggling with their scaly legs. When I built a new hen house and nest boxes (after the last of my old hens died), I was determined to sort out that problem before acquiring new birds. I followed the instructions of the late Katie Thear, author of many books on poultry-keeping. One of her suggestions to help conquer red spider mites was to line the walls, ceilings, floors, and doors of the hen house with a very shiny, slippery human home insulation product, siliconing every crack and crevice in the structure. Apparently, the mites find it difficult to climb about on the white, slippery surface. When all the work was done, I went to my chicken supplier, a long-time exhibitor, now retired, and asked him how all of the birds he had on his premises were scaly leg free. He just pointed up to several huge paper sacks on a shelf in one of his barns. When I looked at him blankly, he walked over to an open sack and showed me the DE. I bought a 15 litre plastic tub of it, for starters, mixed some into the shavings I'd placed in the hens' house and nest boxes, brought the hens home, and the rest is history. I give the birds a scant teaspoon of DE in their dampened coarse meal every single day. I mix some in their dust bath and around the ground where their feeder stands. DE is a desiccant and, as such, the chicken poo is quite dry when I scoop it out daily. The coop never smells yukky.
A couple of friends with hens commented on my girls' clean legs. So, I gave them a supply of DE and they, too, have had great success in eliminating scaly legs.
We over-winter our Shire horses in a vast pole barn. We use cardboard bedding (best bedding I've ever used) and mix some DE into it to help reduce the occurrence of mites. Many hairy-legged horses have on-going problems with feather mites and our experience is that the DE really helps to keep it in check.
And, no, I am not a DE salesperson. I have just had very positive experiences using the substance.
Just to add, I have asthma and have not had any negative responses using DE. I did wear a mask when I was applying DE to the floor and bedding in our pole barn.