Post pounder comments are HIGHLY dependent on the soils you're driving into!

Some have a small auxiliary auger which drills a pilot hole and greatly improves the ability  to drive a post in straight.

You might talk to a local fence contractor and find out which pounder they use in your local (rocky Ontario) conditions. There is no similarity between your conditions and KS, MO or WA.

I have a 3-point mounted Shaver 10 and it works well but on my rocky clay SW WI slopes takes some experience and finesse to get them in straight. I wish I had the Shaver pilot auger option.