In short: I’ve used both the Zwift RunPod and NPE Runn. Out of the two, I preferred the RunPod. However, I had issues with both of them and these days I use a smartphone app (treadmill speed transmitter by BIPR) to transmit the speed data.
I started off with the Zwift RunPod. I found the accuracy fairly good, with deviations up to ± 0.6 kph. However, I wasn’t fond of frequently recalibrating the RunPod to keep the accuracy consistent. I also had issues with the RunPod overestimating my pace. (Great for PRs, not so great for personal integrity!) I also found that the RunPod was quite inaccurate outside the three pace zones I did the calibrations in.
I got the NPE Runn for two reasons: To provide more accuracy than the RunPod and to transmit the incline data to Strava (Zwift itself disregards that data). However, in my experience the Runn did not meet either of my expectations. The transmitted speed fluctuated WILDLY (whether it’s the fault of the sensor or my treadmill i.e the belt being loose or the built quality not being sturdy enough, I can’t say) and I found the transmitted incline data to be very inaccurate. At the price of the Runn I decided to return it and go back to the RunPod.
I also tried using Under Armour smart shoes (HOVR Sonic 4 in my case), but I found the sensors to be the most inaccurate of the three. I messaged UA support, and they told me that the sensors are more designed for outside runs (to correlate with GPS data). They told me there might be a firmware update to alleviate the issue in the future, but I haven’t seen one yet. Great shoes, not so great tracking.
Finally, I landed on the treadmill speed transmitter smartphone app. As you set the speed manually in the app, it matches exactly what your treadmill says. (Of course, you have to trust your treadmill to display the correct speed, which is fine by me) Some folks might not prefer to hit the buttons on your treadmill AND on the app to set your speed, but I personally am okay with it. I had some issues getting the app to work at first (Zwift didn’t pick up the BT signal on two Huawei phones, but it works just fine on an older Nokia model) After the initial issues, the app has worked perfectly. It’s also much cheaper compared to a hardware solution.
In the end, I’ve landed on the following solutions:
Me:
Speed: Smartphone app
Cadence: My UA shoes
My partner:
Speed: Smartphone app
Cadence: Zwift RunPod
Not sponsored by BIPR. 
(In fact, I found their cycling app to be pretty bad)