I worked in a bone marrow/stem cell transplant unit for a few years. It is not a fun procedure for patients. You need to know that the chemo will destroy all fast reproducing cells. Blood cells are one of them. It means you will likely need blood and platelet transfusions after the chemo.
Most people will have neutropenic fever. It means that the immune system will not be able to fight infections and that left untreated these would be deadly. So you will receive a cocktail of 3 antibiotics, usually the biggest hitters, to cover all basis, along with antifungals.
Then the chemo also kills the gut cells since they are too fast reproducing. All mucosa cells in fact, which means your mouth will become raw, all lining of the mouth would slough off, and this would also apply to all the gut cells all the way to the other end. Pain, and horrible diarrhea would definitely occur, not just one day, usually couple of weeks until your stem cells reach the marrow and start making new cells again.
While the mortality rate has decreased for this procedure, it is still a risky intervention and in general patients have no clue how hard it gets.