A lot of powermeters are not EDIT- ACCURATE. Which means that although they work well compared to themselves (repeated measurements), they would be off compared to a 'gold standard'.
I have local friends who I don't get to train with due to scheduling but we follow each other on Strava for years. 2 of them got new Garmin pedal powermeters. These PMs have gotten a lot better from their initial iteration, but I suspect there are still some varying accuracy issues - one guy who has always be slower than me and now averages a full 3 mph less power hour, somehow has an FTP near 300 with his new powermeter, which is a good 60 points above around where mine is (with Assiomas). I can't help but seriously doubt that he has such an FTP in reality ; he only rides 2x/wk and isn't even a racing triathlete!
I have local friends who I don't get to train with due to scheduling but we follow each other on Strava for years. 2 of them got new Garmin pedal powermeters. These PMs have gotten a lot better from their initial iteration, but I suspect there are still some varying accuracy issues - one guy who has always be slower than me and now averages a full 3 mph less power hour, somehow has an FTP near 300 with his new powermeter, which is a good 60 points above around where mine is (with Assiomas). I can't help but seriously doubt that he has such an FTP in reality ; he only rides 2x/wk and isn't even a racing triathlete!