The grate assembly in a Kerdi drain (like most) is adjustable in height to account for variations in tile thickness, but the flange on the drain body itself must first be level. The grate assembly does have a slight plumb adjustment, but not the drain body. The thing is that you're tiling the flange. In a mudbed, you are not with a clamping drain, so slight errors in plumb aren't a huge issue WRT the tile around the grate (but it will still throw off the screw in grate holder!). Because the grate is MUCH smaller than the flange on a bonded flange, small errors get exaggerated...think 12" across verses the grate which (on a Kerdi drain) is 4" square.
The hassle with a liquid applied waterproofing is the variability between two people painting it on...each one will have different coverage numbers. The stuff needs to be installed in two layers, and EACH layer needs to be between the min/max. Too much is as bad as too little, so you need a wet film thickness gauge to verify your technique while doing it. It's easy to get a bit too much on, and then it runs. If it's too thin, you can end up with a pinhole. A uniform sheet membrane is waterproof as long as you do your seams properly. It's sort of like installing wallpaper, but you overlap the seams, or use the (thinner) banding material to cover the seam (less buildup, but it's 8mils thick, so not catastrophic - the banding is 5-mils).
There are numerous companies that make tested, approved showers. In all of them, it is important to follow instructions. They all work, if you do it properly. They each have their own benefits and peculiarities that you need to understand to do it correctly.
In the Redgard one, you have lots of points of failure where you can create an error, and FWIW, their sealant/adhesive is not inexpensive. A Kerdi shower can be done with only thinset. It was introduced into the USA in 1989 when it got first code approvals. That's 28-years. It started in the US with a small building with about 1000sq ft...the main facility is now easily over a million sqft, and there's a second major facility nearly the same size on the other side of the US. Kerdi for sale in NA, is, I think, made in Niagara Falls, Ontario, where they have cheap electric rates from the hydro at the falls.