I am guessing the problem people are seing is with the impedance of the potentiometer in the expression pedal.

I have used a FV-500L a few years back which seemed to worked fine for me.

But I prefer the build sweep etc of the Mission Engineering pedals, I have two and I used to use one with the ModFactor and PitchFactor … it is dual channel.. you toe press to switch between the channels…one used to control the PitchFactor and the other used to control the ModFactor.

Although I have gone MIDI now, so the Mission Engineering pedals plug into my MIDI controller rather than direct into the Factors / eclispe.