I dug fence holes in NJ for a couple of years in college, way back.
Unless the soil was totally free and clear (no rocks, tree roots, clean
fill) we had to use the post hole digger and 5' shale bar by hand.
Never failed with the manual tools. We tried two man and one man
augers and they always got stuck. The only thing that was fool proof
was the auger mounted on the PTO drive of a farm tractor. Nothing ever
stopped that. Then if you are digging on blast fill along an
interstate like we had to once you'll need a trailer to pump compressed
air to the 30, 60 and 90 lb. air hammers.

I built a fence in my back yard last year, about 300' of it using a 5'
shale bar and $20 post hole digger. I have wet/damp soil so the
soil/clay was like butter. Took about 10-20 minutes per hole. Easy
compared to what I remember years ago.