I personally have never used PayDotCom, but I love Clickbank as a retailer. They have a fairly seamless system with decent stats scripts and I don't have to waste time calculating or paying any affiliates.
Systems will always win over human error when it comes to payment tasks!
I have noticed some very lame excuses for refunds also...but there are just people like that everywhere, you can't let it discourage you. Just stay true to your path.