The only thing that is a real PITA for loading in the waypoints manually is getting any type of Oceanic waypoints entered in correctly with the correct LAT and LONG. Other than that, a more simplified route with just airways or direct waypoints is easy. As for how FMC’s function, it doesn’t really matter too much whether you use an A320, 737, 787, etc… Once you’ve programmed one, you can more or less program them all with some specific differences.