Smart Band 2, is sleep tracker good?

Hi, I've adquired an Smart Band 2 and I was thinking of using it to track my sleep.

My doubts comes with the times its registrating for REM sleep
For example, last night I slept 8:11 hs, with only 5 minutes of REM (according to MI Fitness).
I do feel quite well rested and I do not think it was a bad night at all so I'm kinda confused.

I know REM is not all when it comes to sleep quiality but its an important part. I always have like 30 minutes (gotta get those numbers up lol) but 5? seems just... wrong or impossible lol xD

Is the sleep tracker good and I'm fucked up or its not that precise?