Before I knew about cryptocoins, I ran a lot of GPU's for BOINC distrubuted computing projects. There are/were(?) heaps of CPU based Prime number finding projects but none of them ever used GPU, so I am agreeing that branching is not suitable for the hardware and that is why no one ever bothered to make a program to mine on them. For this reason I suspect we might never see a GPU or ASIC prime number miner.