This heart monitor is accurate or as close to accurate as you can get when it actually works. The Mi Band 2 has issues with monitoring your heart 2 out of 5 times. After 20 seconds the monitor will display an error on the screen because it failed to read your heartbeat.
There are times the monitor will stop recording anything and refuse to sync up. The only way to get the monitor working again is by restarting your phone. The monitor will always display the data on the phone app so you can see what is happening. But the amount of data lost, the reading not working, and the sync between the monitor and the phone app, it isn't that reliable is you really need to monitor your heart all the time.