"I honestly don't believe that 99% of the staff at both local building stores I went to would recommend the wrong product, "
If you are talking about places like HD and Lowes they are famous for many of the employees not knowing what they are talking about.
"My point is that cement board too will fail under moisture eventually,"
No, you can soak an of the CBU's in water forever and it won't fail.
It is completely water resistant. But it is not waterproof. That is it will allow moisture through.
That is why you want to install a barrier behind it to product studs and insulation behind it.
There are also different trowel on, paint on, and films that can be applied over the backerboards (of any kind) to water proof the assembly.
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