There are two ingredients to success:
1. Coin algorithm. How compatible is it with FPGA.
2. Qualification of the programmer.
Very often, stupid programmers create an aura of impossibility of any process. Then a skilled programmer destroys that aura in a day. But a skilled FPGA programmer is expensive and usually does not deal with crypto
FPGAs are expensive and not produced in large quantities. Lots of export restrictions here too. This all complicates the development of a commercially viable device.