The bathroom is a route, not a collection of fixtures. The route begins at the door. The width of the door, the direction it swings, and the clearance it leaves in the room behind it decide whether the route into the room works. From the door, the route moves through the room to each fixture: the toilet, the sink, the shower. The path between these points has to be navigable. The floor underfoot has to be continuous, level, and provide grip. Only after all of this is true is the shower question asked.
The shower is actually the last question in planning a barrier-free bathroom. By the time the shower is specified, the room's route has already been decided. If the route works, the shower can work. If the route does not work, no shower specification will fix it.
The door is the first detail. A standard hinged door opens inward into the bathroom, which takes up floor space inside the room. A door that opens outward into a hallway does not consume bathroom floor space. A sliding door takes no swing space at all. The choice of door type has an immediate effect on how much floor space is available for the route through the room.
The path between fixtures is decided by the positions of those fixtures and the floor between them. If the toilet is positioned in the corner, the path to it is shorter but harder to approach. If the toilet is further out into the room, the approach is easier but the path through the room is longer and more complex. The same logic applies to every fixture. The route is the aggregate of all these paths.
The floor surface has to be continuous. A change in floor height — a threshold, a lip, an edge — is an obstacle. A continuous floor that slopes or is not level is manageable. A discontinuous floor, even at a small height difference, is a barrier. The grip of the floor matters too. A slippery surface is harder to navigate. A floor with grip is more stable.
What has to be true of the rest of the room before the shower is usable at all is that the route to the shower works. The door opens, the path is navigable, and the floor is safe. The shower itself then becomes a detail within that working route, not the determining factor of whether the room works.