I think it's important to get one thing out in the open - Liver Flushing can hurt, it can suck, it can make you wish you never did it, and it can put on your knees praying the Porcelain God, but I can't agree that unpleasantness indicates a lack of body-aptitude. It is what it is - and that changes with all of the preparation factors, etc. and how one's GB has progressed. Nothing hurts worse than stagnation that cannot move, but it can't stay stagnant forever. Learn as much as possible before proceeding.