I miss my smart watch waking me up outside of detected REM sleep.
On the Microsoft Band you could set a time window where the alarm would go off - say between 0700-0800. If you're in REM sleep at 0700, the alarm stays off until you naturally rouse, or 0800.
I've worked as a sleep scientist for 7 years, and the idea of not being woken out of REM is such a neat idea, and yet no other watch seems to do it.