With any manufactured cement board/etc panel, the walls will only be a flat, level and square as the framed enclosure.
Water tends to get through them over time, rotting the framing.
Cracks typically develop in the tiles or grout in the corners or on the seams.
It's almost impossible to use a cement panel on a shower wall that has a tiled shower floor with long lasting results.
For the trouble that it takes, you are time ahead for appearance and longevity to just float it.
( do it right the first time ! )
I like to use hardie backer on floors only.
I go through quite a process to get the floor flat before I set any tile, though.
Subfloor prep as well as backer leveling.
Floating floors is not generally practical on a remodel, as adjoining floors and exterior thresholds do not allow for a 1-2" thick floor.
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KT