Hi, Patrick.
Most pros have done some hybrid or another with liquid/sheets/different drains. But the thing that makes many of us the most nervous is using a liquid waterproofing on a pan. If the stuff is applied to the proper thickness and all the details are taken care of properly, it's going to work. But it's sooooooo easy to not be thick enough in one or two little spots. And a shower is no place for skimping or being lackadaisical on waterproofing. The stuff isn't magic. You need a certain thickness, but it's not a "thicker is better and more waterproof thing". Too thick equals not enough support. With a mud pan surface, there's a lot of minor inconsistencies. So, it's super easy to apply the stuff too thin or too thick. Walls are a very different story. In my opinion, most pros are nervous using liquids for waterproofing a pan and perfectly comfortable using it to waterproof walls.
One think you might not know is that the pan would need to be completely dry before you apply the RedGard. That means waiting 28 days. Somebody can correct me if they've changed the requirements and reduced the times. If the requirements have been reduced, I'd want that in writing.
