Nailing down hardibacker on walls
Depending on what is behind your backer you need long roofing nails.  I use HB 30-40 times a year and and I use the HB screws and long roofing nails.  For shower walls that are a little tough I will double predrill for my backer screws.  I will use a small bit for the hole then I will through each hole and use a larger bit.  The larger bit I just give a tap into the board - about a 1/16th of an inch.  This will allow the head to sink in well and also keep my backer from breaking on the corners.  It might seem like a lot of work to double predrill but when you need to end up with flat right angles... well you will spend more time screwing and unscrewing - and backer screws are really good at pulling your board out 1/8" to drive you nuts.  

Another note on the roofing nails.  I strategically use them.  They are fine to use, they are easy, they are faster... until you need to take the board out or add a shim.  Then you are screwed.