Digging Post Holes in Hard Ground / Rock

I wonder how the Bel Tec would do in the following two circumstances?

1: Dirt and field stone
9"-12" auger in dirt with field stone ranging in size from 2" to 14" regularly getting hit.

2: Solid Blue slate
We're at the edge of an old morraine and occasionally have to dig through solid slate (ouch)

__________________
When faced with issues in life, where do you look for the problem; out the window, or in the mirror?

...............Bel-Tec will work very well where the subsurface rock is fractured , as the auger will leverage one piece against another and simply fracture them into smaller pieces and maybe even auger them out of the hole . OTOH , if drilling in a basically unfractured layer of something very hard I'm afraid that the hardened cutting tips will shortly become dull and need REsurfacing and sharpening .
................I'd go rent a Bel_Tec digger from a rental place and actually try it on your tractor , OR just rent a tractor with one already installed BEfore purchasing one and then finding out it isn't up too the job . , fordy