Why is Easy Sand not suitable as a skim coat?
Setting-type joint compounds set by chemical reaction with water. When a very thin coat is applied to unpainted drywall (such as the thin skim coat applied to new drywall needed to achieve a level 5 finish), there is a risk the board will suck the water out of the mud before it has a chance to chemically set. If this happens, it will revert back to powder and fall off the wall. Don't do it.

If you're talking about skimming an ugly old wall to make it smooth, Easy Sand is fine, maybe even ideal. Just remember that old walls will have multiple layers of paint that could be peeling or delaminating below the surface. As long the old surface is stable, go for it. Just keep it thick enough that it's not translucent.