Sustain is not a variable function on these keyboards, regardless of which pedal you plug in so a simple switch like the FC5 works perfectly well, but the piano type FC4 just feels much better. The FC7 is ideal for all the pedal functions which are variable, like volume, pitch bend, etc ....
I have a CVP which has three piano pedals built in, I have the right one permanently set for sustain, the centre one for tempo tap in, and the left for style start/stop. On the CVP you can plug in a 4th pedal, and I have an FC7 plugged in and set for pitch bend on instruments like guitar, harmonica...
The tempo tap in and sustain are the two I use all the time, the others are only occasional.
Mike