A Pi si just about fast enough to control the data coming to and from a few ASICs. It's never, ever going to have enough CPU power to even get a GPU working.
It's like raising London's Tower Bridge using a cordless screwdriver.
It runs several more than a *few* (I have had 21 block erupters on 1 Pi with no issues).
FPGA would take over the processing the RPi would end up as network translation and other low demand tasks.