I have Easy Sand 90 joint compound. Its label says not to use it as skim coat. Why can't I use it?
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.
The easy sand is a "light-weight" compound which is why it sands so easily. If you ever sanded this type of compound you'd see why it shouldn't be applied in thin layers. It sands off very easily. If it is applied as a skim coat it wouldn't last 1 or 2 passes with your sander.
You will see this frequently on all types of wall patching products. If it has "light-weight" any place on the label chances are that it will state someplace in the instructions "not to be used as a skim coat".
Setting-type joint compounds don't tend to feather out as well as premix compounds, especially if they're not mixed extremely carefully. They're likely to leave a ragged edge requiring much more sanding than should be necessary.
Also, per the spec sheet for the product:
Do not use setting-type joint compounds for thin skim coats. If setting-type joint compound dries before it sets, bond failure may result.
It isn't that you can't use it for finishing, it's that it's not an ideal product.
I don't know what you read. From Quick-set.info:
Excellent for all phases of finishing, including taping, finishing, filling, patching, skimming, laminating and even texturing.
However, it is better to use lower drying time Easy Sand compounds, and you need to make sure you've prepared the surface if needed.
Also, "Easy Sand" is not easy to sand, especially the 20 and 5 minute. That's why you have "topping" compound.