a one-piece tub & shower combo, most cannot be removed in a single piece unless you have a huge bathroom. They were installed during construction. If this is your case, get out the jigsaw a cut it up.

Similarly, a new one-piece is difficult/impossible to get into the room. Most opt for a two or three piece installation. I agree that a fibreglass tub can be a problem if not correctly installed. The floor needs to be built up to contact the bottom of the tub. If you don't the tub will feel spongy and the drumming from the water will be a PITA.

IRT to floor, for tile 3/4" subfloor is good but for best results, plywood instead of flakeboard and nail it every 4".