The cool thing about using the proper amount of acrylic modified thinset under the backer is, once it cures, the backer is laminated to the subfloor as well as mechanically fastened. No air spaces that can move, potentially causing cracks in the grout or tile above.
Nails or screws, it's your choice. I've done both. Neither have ever failed on me.
I've installed 10's of thousands of square feet of this stuff.
Screwing the backer takes about 10 times as long.
From what I've seen, most people, pros included, do not take these steps seriously.
They skip the under-backer thinset, joint taping, priming and leveling.
At least it makes it easier for me to tear it out and redo it
KT