For my sins I'm in the bathroom bashing game & have been using the same spreads for a number of years.Most of the rooms I tackle need tiling, so after rip out, I ask the for the room to be bonded & skimmed. They always deliver a smooth finish, but on quite a few rooms of late, I've had to use 3 or 4 extra bags of tile addy to fill the voids & make it tileable. Is to much to ask that they use a level or straight edge to get it flat & level? See pic.