Beyond walls, there's entire rooms, provided your client wants a lot of identical factory-finished rooms small enough to transport. Doesn't really work for your average private housing client who wants big open plan areas to add their own decoration to, but it's ideal for chain hotels and student/social apartments. That's where the construction speedup is more noticeable because the unit fitouts can take place in parallel to site prep and foundation works, and where you also get economies of scale and better QA from fitting lots of copies of the same bathroom on a production line.