too much dust. dangerous.
these boards produce a lot of dust, fibrous and cementitious. Dangerous to health. A big wet saw ($1000) will cut it with little dust. A small wet saw will cut it with little dust, but be hard to manouver.
Score and span is the best way to cut it without buying the big wet saw. Some dust is made; it can be reduced by keeping wet sponges on hand and leaving lots of water on the board. Any power tools will throw a thousand times more dust up into the air. Many people have written about how bad that kind of dust is, and how it gets into everything.
2.
About using Fiberock or Hardibacker on a shower wall: technically speaking, they are not waterproof enough to be shower membranes all by themselves. See the web site. Call the manufacturer. You still need waterproofing, either a liquid substance which hardens or a sheet material which you thinset on.
For bathroom floors there is no requirement for a waterproof membrane.
david