I can confirm the M-Audio EX-P works well with Helix, allowing for a smooth full sweep from 0%-100%.
My settings: Switch on pedal bottom panel is set to "M-Audio" (not "other"), range wheel on pedal is set to full (full counter clockwise turn) and polarity set in Helix to "normal".
The one thing I have learned to mind is to set the above "parameters" on the pedal, before you plug it into Helix. If you're playing with the pedal switch and the range wheel while plugged-in, you get strange responses (no response, or limited ranges). The reason for this I believe to be the auto-calibration process. It's only initiated by certain actions, and for sure it is when you (re-)plug-in the pedal. If you change settings shortly after plugging in, you can get strange things. I was always able to solve things by disconnecting and reconnecting the plug, forcing a re-calibration.
Obviously, once everything is set, not need to touch things anymore, and the pedal works like a charm.
Admittedly, not realizing the range wheel was somewhere in the middle when the pedal came out of the box caused some confusion on my end before I realized what was going on (and the M-audio manual is not even mentioning this wheel). Holding the pedal in my hand and accidentally affecting the range wheel while pushing down the pedal spiced my confusion, as you can imagine ;-)
I agree to all forum comments on "plastic" build and design, yet price/quality range for this pedal is unmatched. It does well what it needs to do, and happy to see how long it will last before breaking. My gut feeling says it will last if you treat it with respect. If it breaks, it was worth the gamble for 20€ shipping included. I am just flabbergasted at the prices charged for a quality pedal, comprising nothing more than a metal case holding a hinge and a pot.
Disclaimers: only have the pedal for a week - still plenty of time for the pedal to break early.