Hardibacker is a VERY "thirsty" material that sucks the moisture out of the mortar VERY quickly and I don't like it. Cement board is a bit better, but you need to spray either one before applying the mortar so that it doesn't "skin over" before you can get the Kerdi up. Personally, I like standard drywall. It works well, is what Schluter recommends and I've never had an issue with it. Look at it this way; Withe Kerdi, you either trust it or you don't. I trust it and have never had a callback.

It's not a matter of trusting the Kerdi. The problem is on the back side of the drywall, which is not protected. I used to "trust" the Kerdi and drywall until I had a shower that the valve broke inside the wall cavity four years after being put in. Needless to say, the drywall turned to mush. That was not a fault of the Kerdi, or the installation. Not even the plumber's fault as it was just a bad valve. If cement board had been used, most likely the damage would have been much less that what I have to deal with now. You are wise to use a cement board. I would avoid Hardi, as Avanti is right that it is thirsty.

If you want a cement board, use Durock or Wonderboard. I usually use Durock. DIYers on this forum seem to have nothing but problems with Hardi.

If you want added info there's an old Kerdi thread here. Good luck with your project.

Thanks to all for the feedback. BTW the older Kerdi thread referenced
was excellent. There is definitely skill & planning required to do
it right. Do the prefab Schluter niches like the double unit 12 by 28
shelf have a built in pitch to the bottom and middle shelves to drain
off water as shown in niche that you built in that thread?

They are square. You provide the pitch during the tile set. The niche' intermediate shelf isn't actually attached, to enable specific placement for each unique tiling application.