I can tell you from personal experience that the quality of the hole you drill will be in direct relation to the soil your drilling. Here in north Idaho a 9" drill works OK, but a 12" is a real pain in the $$$$ to use. most if not all small tractors don't have power down on the 3 point hitch, so they just tend to float over rocky or dry clay soils. Some makes (like my Speeco.. have a hydraulic down-force assist that helps. If your going to do this for a trade/business, most people use a front loader arm mount post hole digger that can be powered down, either on a skid-steer or a compact tractor. The skid-steers have higher hydraulic pressures and volumes to drive these units. Most tractors require a pto operated pump to power a front mount drill properly. There are some low volume drills available, but i don't know how well they work.
These front mounted units are kinda spendy though. If i had to do it all over again, i would have bought one years ago.