Well said, that's real life when it comes to any old house with any repairs.
I call them a Pandora's Box. Open up the walls and there's just no tell what your going to find.
Anything done "cheap" in a bathroom will come back to bite you on the butt, and may end up costing you more in the long run.
It's a one shot deal, done right, it will last at least 20 years.
Trying to save money by not changing an old shower control valve while everythings open is one common mistake, what do you think it's going to now that it's closed up behind a wall when the parts are no longer avalible or the body is worn out.
Cheap flooring or the wrong flooring for a bathroom such laminite, cheap sheet goods, peel and stick tiles, will always fail and have to be replaced.
Not using tile board, not waterproofing the tile board, cheap partical board subflooring or underlayments, partical board vanitys. All will end up costing you not saving.
At least 90% of the old houses I've worked on had uneven walls, undersized and to long a span on the floor joist, unlevel floors, old steel pipes, all needed attention to do the job right.