I use a Source Audio Reflex expression pedal with an H9 and PitchFactor (along with everything else).

The MIDI implementation is second to none.  The LFOs alone certainly bring the Eventide pedals to another level.  Almost too much.  That is, it can send more MIDI messages more rapidly than the stompboxes can comfortably handle.  It’s not a problem once you feel out where the upper limits are, and back off a little.  In other words, you’re not going to want to blast out 3 LFOs on three distinct parameters at fast rates.  By no means is that a deal breaker, but it’s something to be aware of.

The promised Reflex MIDI editor app is still not available, and has been reported by Source Audio to have no defined timeline for release.  So don’t count on that, if it matters to you.  I’ve read that the ‘Soundblox Pro Bass Envelope Filter Sequence Editor’ app can create User presets for the Reflex, but I haven’t confirmed that yet.  AFAIK, all Reflex programming will have to be dialed in manually for now.  Not that difficult, a bit of a learning curve, but tedious.  It would be a lot faster with an editor.

As far as the expression pedal implementation goes, it’s just as powerful as MIDI, and arguably better suited here.  I’ve experienced one quirk that I haven’t solved yet.  It burns 9V batteries with a TRS cable inserted; whether or not the Power switch is engaged.  I see a low voltage ‘bleed’ reflected in the on-board LEDs.  It has to do with my current DIP switch configuration [Normal], and perhaps my target device (through an EHX 8 Step Program before out to the actual pedals).  Not a problem with an attached power supply, or if you remember to unplug the cable (which I never seem to do).