My trick for not splitting the ends of trim with gun nails: notice which way the chisel point on the finish nails is aligned, and nail it so the chisel exerts force on the length of the grain, not across the grain. That makes the difference in 90% of cases where it would split one way but not the other. And yeah, with oak, don't nail too close to the ends and use smallest nails that will hold - or hand nail - like they all said above.
When you hire someone to do stain grade trim, and they accept, there's a basic level of quality that I hope everyone would take as an unspoken agreement. As a finish carpenter myself, I don't settle for anything that doesn't look pleasing or at least okay to the naked eye from a normal viewing angle. I mean, if you're laying on your belly and examining it, you can see little flaws in almost anything, but if it's really as bad as you say, then that's not good work. Can you get him to fess up to that and fix it, or if you feel he's really not capable, not pay him in full for not doing what he agreed... hopefully you can discuss it in a civil manner.