The other cost of heat treating is the additional resources (including time) required to do it. It's been my experience that without a solid form, the bow will warp. Without a way of producing gentle, flame-less heat, it's too easy to scorch the stave. So you need additional stuff to heat treat, along with the time. But if you have that stuff and can afford to invest the time to do it right, then heat treating seems to have no drawbacks that I've found.