If you have bad thinset, nothing will work!
Kerdi is hydrophobic...it literally repells water. Tests have shown under pressure (and a shower pan is not under water pressure), water migrates a maximum of 1/4", and the spec calls for all overlapped joints to be at least 2", so there is a huge overkill. Water does NOT flow under it, if properly installed. You install the stuff sort of like wallpaper...thinset, then place the sheet, then press it flat into the thinset, work out any air bubbles, and excess thinset. And, since it is entirely waterproof, you can build your walls out of drywall (which is the recommended substrate), which is cheaper and much easier to install and cut than cbu.
For the pan, you can use their preformed ones, or build your first layer of deckmud just like you were going to do for a conventional liner. Then, you install the Kerdi. One less layer to build and shape. This means your step into the shower is closer to being on the same level, which is more comfortable, and it make it easier if doing a barrier free shower since you dont' need as much depression or elevation, if elevating the whole room other than the shower. It's faster to install, too.
I took a class at the TCNA headquarters where they had a cardboard box covered with Kerdi and thinset. they used it as a drink cooler. The one I saw had been used daily, filled with ice, and drinks for about 6 months. the paper was entirely intact with no water stains. If you take a square of the stuff, roll it into a cone and fill with water, no water will come out the point...it's good stuff. their drain is the easiest to adjust, and with a surface membrane, you don't have an inch or more of porous deck mud that can get damp...showers dry out much faster.
