Backer board is not water proof and it will mold. Read the manufacturer data, it will wick 3x as much water 3x as far as drywall.
The old way was to install vapor barrier behind the backer.
I prefer to install the backer and redguard it. redguard is a brush/roll on water proofing anti fracture membrane. Very easy to use.
I actually prefer to skip cement backer entirely on walls and opt for denshield with redguard. The denshield lays smooth, cuts with a knife, is flush with the sheetrock, doesnt fall apart when you screw the edges, is already water proofed on the face, and it can be seamed with caulk instead of thinset and tap.
For full steam showers i go schluter.
If you tape use the correct mesh tape. Humps will make tiling very difficult. Do a little research on thinsets, if you use the wrong stuff you are going to have problems. They are not all created equal and for use in every application.