Traditional (Under) vs. New (Topical) Waterproofing Systems
One the biggest mistakes I see being made is the confusion between the two main systems for waterproofing a tiled shower pan. While the new systems are becoming more and more popular among professionals, there are still a lot of traditional pan liner and other traditional weep pans going in. Home Depot sells A LOT of pan liners/Oatey drains to DIY's/Handyman, and the problem lies in the installers not being properly educated. Some of the problems that I see reported to me are:
A pan liner with tile applied directly to it
Pan liner/Hot mop material is NOT intended to have thinset bonded directly to it
Screws through the pan/liner hot mop when screwing cement board over the curb/bottom of the wall
No fasteners are allowed below 2" of the top of the curb
Redgard or other Topical Waterproofing to cover tears/cuts/or screws in the pan liner.
Topical waterproofing such as Redgard, HydroBan, or Kerdi in general require a bonding flange drain, not an Oatey weep drain. Screws or fasteners should not be through the curb when using a pan liner. If cuts are needed in the pan liner, the appropriate Oatey pan liner cement should be used to repair.
"Going over everything" with Redgard when using a pan liner.
Going over a mortar bed pan with Redgard can create a "mold sandwich". Traditional weep systems using a pan liner are designed to allow water to flow through the mortar bed and out the weep holes at the base of the drain. If you cover it with Redgard there is no way correctly bond the Redgard to the Oatey drain, which will allow water under the Redgard and become trapped in the mortar bed. Without fresh water to flush the mortar bed, a breeding ground for mold is created.
When using a Topical Membrane such as Kerdi, Hydro Ban, or Redgard a "Bonding Flange" drain such as FloFx should be used:
There is no pre-slope (slope under the final slope of mortar) needed for bonding flange topical waterproofing systems since there are no weep holes. These top sealed systems do not allow water to go through them, so there is no need for a slope under the mortar bed. Since the waterproofing is directly under the tile, screws for the backerboard can be placed anywhere on the walls or curb, and the waterproofing is placed OVER them.
Confusion between the two systems can happen when people see a topical system (kerdi) on TV or the internet and they go to Home Depot and pick up a pan liner/Oatey drain and think it is installed like the topical systems. The people working at Home Depot may be similarly uneducated and just tell them to just "go over everything" with Redgard when they are done.
Any of those wallboards are good