I think big differentiator is that it is easier to get predictable latencies with FPGA where you control almost everything, compared to general-purpose PC which

I think big differentiator is that it is easier to get predictable latencies with FPGA where you control almost everything, compared to general-purpose PC which is not really that well optimized for hard real-time operation. So I believe "race the beam" style things are more easily accomplished with FPGAs, and also having tight audio-video sync. Although the PC emulation scene has been also doing some fairly incredible things too.