It depends on what aspect of tiling you do most, but some are harder than most other trades, plastering is hard on the body, especially ceilings, but it's a skill that is only repatition to get a decent finish, small ceramic tiles aren't to hard on the body and relativly low skilled most of the the time, but doing large commercial or domestic floors at speed are very hard on the body, and it takes alot of skill to get them right working at speed, same as larger tiles in smaller rooms, a bathroom in 900x 900 12mm or bigger porcelain will give you a work out and requires alot of skill to get it all right as these rooms tend to be high end and have alot of niches, and other features plus pipework etc and cuts on show, the technical aspect has overtaken most trades now and only getting more need and expansive, the popularity of mitres etc has added more to the skill set, and stone work takes the pyshical, technical and skill to higher levels. But the long and short of it is, the higher end of any trade is hard as the detail needs to be perfect tiling as it is now is a in a new place and is only getting harder, still don't know if I'd say hardest in any one area but in an all-round mix of skill, technical knowledge and psychical effort it's up there now