Drywall is an industry approved method according the the TCNA when used underneath Kerdi. You do NOT want them to mud and tape the seams where you are going to install the membrane, the thinset and membrane will provide that function. When installed properly no moisture from the shower will get beneath the membrane and the drywall stays dry, retaining all strength it had upon installation.
Installing cbu, and I'd guess (haven't looked recently) the fiberglass faced backer boards, are also one of the listed approved backers. Because you have a lot of area to spread the thinset on, if you use cbu, you want to wipe it down with a wet sponge to do two things: remove any dust and to provide some moisture so it doesn't suck it all from the thinset, making it dry, which is then hard to embed the membrane's fleece into properly.
Schluter has decades of history of using drywall, but they also list other materials that are suitable...go with the one you prefer, but in any case, done right, none of them are a problem waiting to happen. If you had a leak from say the plumbing, cbu would likely survive, drywall might not depending on how bad and how long it went before discovery, so that is a strike against that...but, consider all other areas in the house where there's drywall and plumbing behind it...do you worry about that, too? Plumbing does not magically leak. Obviously, it can, but it's not all that common unless disturbed or incorrectly installed (same with the membrane). Do it right, do it once, and don't worry.