I asked my vet about this once, thinking of using it instead of bute, his response was pretty much as expected, it will not have been tested in the same way as a prescription drug so there will be no definitive answer, the liver will still be potentially at risk from it's use as will the stomach, that most horses on bute will die/ be pts from other conditions before the potential liver damage becomes an issue, unless the liver is already compromised in which case you would not want to use devils claw anyway.
I have the lad (14yo cob) on Devil's Claw and have done since the spring. I definitely noticed an improvement within about 10 days and the only bit that didn't improve as much as the rest of him were his hocks, which on closer investigation proved to be showing early arthritic changes. Hocks were injected and started him on a joint supplement (vet's own) which has a good dose of chondroitin sulphate. We've not looked back.
According to the vet, chondroitin sulphate is what joint fluid is mostly made of, hence being helpful.
I've kept him on the DC as I don't know if he's a touch arthritic anywhere else and as above, he's likely to expire from a variety of other causes before any liver issues from the supplements.