I agree with Steve. This is tough to answer because there are a lot of products and variations out there. The problem could be variations in the amount of movement that the finish will take without flaking, peeling or cracking. This might show up immediately and it might take many substrate movements. Everything moves. You may not be able to see the movement which is caused by temperature and humidity changes, but they do occur. No problem with liquid stains and other non-film building products.
One other point is that the cans of shellac and oil based poly say to not use then together. The reason is that there is a little wax in the shellac which may not allow a good grip for the poly on top. I have done this many times and not had a problem. I probably will now.