After three months of running on Zwift with Runn Treadmill sensor and a handful of hours tinkering with mounting, calibrating, and replacing belt stickers, I finally got it to work as advertised.
The roller coaster of experience went something like this: when ordering it, I was expecting a 5/5 star product as advertised. But after setting Runn up and facing issues with stickers, mounting, calibration, I would have given it 2/5 stars then and there.
Finally, after multiple rounds of trial and error, I got it to work correctly and accurately, so now I would give Runn 4/5 stars. I'm taking away one star because of a flimsy mounting bracket and belt stickers that don't stick.