Smart watch metrics - how accurate are they?

I was curious about how accurate my apple watch is, so I did some digging and found this video: https://www.youtube.com/watch?v=fDj2kYIF-sE

In this, they try out four devices: Apple watch, Mi Band, Fitbit, and a phone in hand.

It's worth watching in full, but here's a summary:

1. Steps
   • In general Apple watch is close. Phone does the best
   • If your hand is fully steady, Apple watch counts 0 (in the video he's carrying a glass of water). Fitbit and the phone do better.
   • Apple watch does better when you are walking weirdly too. Fitbit/mi band and phone all overcount in these cases.
   • Fitbit overcounts a lot. For eg, in this video, it counts steps even when driving.
2. Heart rate
   • Benchmarked against an electric measure strapped to the chest.
   • While running
     • All of them mostly do good, except in extreme cases (sudden change in running pace for example).
     • In the case of a sudden increase, Apple watch comes the closest to real heart rate.
   • Throughout the day
     • Apple watch's frequency of recording is 4 minutes, compared to Fitbit's 8 seconds & Mi Band's one minute. This means that Apple watch misses a lot of cases.
     • Mi band is the least accurate.
   • Sleep tracking
     • All watches work well in this case, as conditions are ideal. Avg error is 1.8 bpm for Apple & Fitbit, 3.4 for Mi band