I was using the muin when I first got the power meter. I did do a few tests, but not too many (I don’t think I kept the data). I found the muin consistent, but the accuracy got worse as the power went up (but consistently worse). So it was reasonable up to my threshold but once into VO2 max zones it was further out. My muin FTP was circa 260 at that time.
It kinda threw me a bit when I got a power meter, as I was able to do the 20 minute tests and threshold workouts like lamarck based off the muin FTP with the power meter, but not the VO2 Max stuff with the power meter.
On reflection, it probably contributed to me not trusting the ramp test with the power meter initially (I was a beta tester). I put it down to the ramp test when it was probably the lack of accuracy of the muin’s power curve.
tl;dr it’s consistent not accurate, and accuracy is worse as power goes up!