US's refund process is not automated; it can sometimes take a while to get your money back (45+ days). Given the high CPM (it is rare when the Multiplier offer comes out at less than just buying the miles when they go on sale), the high cost of a refundable ticket, and the amount of time your money will be tied up if the refund process isn't smooth, I'm not convinced that it's not better to just buy the miles outright.
FWIW points.com is in charge of this transaction. The price goes up as the number of miles goes up. So while there are some routings that are more lucrative than others, it's generally only a few miles difference; then you're bumped up to a higher price category.