Tile can be repaired. Surrounds get replaced.
In my 100 year old house in the deep south moisture is always a problem. My remodeler installed a rubber barrier, I think it was called a Schluter pan. I have seen a little bit of mildew on the grout, as expected, but grout is not part of the waterproofing anyway.
I think for grab bars, it’s necessary (or at least preferable) to brace the wall behind the finish surface, such as with a 2x8 board perpendicular to the studs. Once that’s in place, you can drill through the finish surface (tile, vinyl or whatever) to secure the grab bars to this board. I suppose you can anchor the grab bars to the studs, but you’re not likely to have studs placed correctly, and it’s also not easy to locate them under the finished surface.