I believe one big advantage is that most FPGAs can have multiple clock domains. So while everything on the Propeller is tied to the system clock (and the system

I believe one big advantage is that most FPGAs can have multiple clock domains. So while everything on the Propeller is tied to the system clock (and the system clock is hopefully fast enough that you have the resolution that you need to do the right thing at the right time), FPGAs can simply use clocks running at different frequencies.