I looked at FPGAs, but that's not just a learning curve, but a whole new world to learn. I started reading about Verilog and decided that an FPGA was overkill for what I wanted to do (essentially controlling a few signals at 5.9MHz for an Ethernet emulator). On the other hand, for the Xerox Alto disk emulator, Carl (on the restoration team) is using an FPGA.

Is the disk controller using an FPGA because that's what Carl was most familiar with or is there a technical reason? Does data come in too fast for the PRU to keep up or are the timing requirements too stringent?

Carl is using a FPGA because that's what he is most familiar with and he's built disk emulators with FPGAs before. The Alto's Diablo disk provides data at 1.6 MHz, so the 200 MHz PRU could probably keep up. The FPGA can make the timing more exact, and Carl likes to make sure all the signals have precise timing. A complication with disk as compared to Ethernet is there are a lot of signals to deal with (track address, sector address, status signals, read clock, read data, write data, unit select, etc). He goes into details at his blog: https://rescue1130.blogspot.com/

If you can use the PRU instead, that's a lower barrier to entry - in some ways. For stuff that needs to be more or less +/- 1 msec in latency, an Arudino or might be an easier choice, dependent on size, weight and power.

Th real problem with the PRU is that its more or less single vendor.