I’m afraid that’s not quite true about calibrating the RUNN. Unless you’ve been extremely lucky that the factory calibration matches that of your treadmill perfectly.
The RUNN has it’s default calibration but you would be amazed at the variation is speed of running belts. Different motor produce different speeds and each foot strike can have a major impact of the belt speed. Commercial machines are less likely to fluctuate than home use ones.
The RUNN will have a tolerance where the speed will fluctuate +/- 0.3mph of the treadmill speed once calibrated. There is no power smoothing so when you are running you will likely see your speed on the HUD bouncing back and forth.
I’ve liaised with NPE and they advise not to calibrate in game but to use the Configurez app to calibrate the RUNN to your treadmill. The app advises not calibrating whilst running but this is a bit of a health and safety things and from my experience you get much better results if you calibrate at around your normal running pace.
That said I still get mixed results as I get variation in speed at the start of my run when everything is cold to when things are warmed up after a few miles. If you are not data obsessed then it’s not an issue. As James says the RUNN is better than the foot pod but probably not as good as a Stryd but then it’s over half the price.