Silly amounts of speed is not always feasible with FPGAs. If your design requires a lot of interconnections, routing the signals can introduce unexpected delays. The number of roadways is limited; and at each crossroad, there can be only one input signal.
Yes, the companies will give you examples of GHz serial communications, but that's only two wires in each direction. Your PCB will limit your speed capacity.
When you want to control several things with precise timing is where FPGAs can shine. No interrupt latencies, no need to piecemeal multibit I/O one-byte-at-a-time. No need to mask bits, no need to OR bytes together. Just control and sense as many signals as you can; with as many counters and state machines as the chip can handle. Your I/O is as wide as the number of available I/O pins.
Design flexibility is another reason for choosing FPGAs. If your requirements change, but your I/O pinout doesn't, then there's no need to create a new board. If your design can benefit from eliminating a CPU, then this flexibility is potentially a winner. If the logic becomes complex enough to be simplified with a CPU, add a CPU "core" to your FPGA design and write firmware for the CPU.