I use them for my guitars. They work great. I use the clear and simply lay a coat of lacquer over them to seal the ink, otherwise it will bleed when you dunk them in paper.
I use an older HP inkjet printer with no problems at all.
They probably don't need to be, but vintage replica guitar builders use DH's trick.
The original Fender headstock decals were placed after the final clearcoat had dried, and were prone to wear. Later Fender "sandwiched" decals somewhere in the middle of the clearcoats. Some vintage replica builders lacquer the replica decal on the card prior to trimming to add scratch resistance. Then they apply that decal to the finished headstock which keeps a visible vintage decal edge.