Well, I own a fitbit charge 2 and one reason I bought it was the vibration alarm clock. It's a very light watch and I don't mind having it on my arm while sleeping.

You define multiple alarms on your desktop/mobile, for specific weekdays, synced with bluetooth and you can disable/enable each one of them on the watch, which I really like.

But there are some minor problems with this watch:

1) the vibration alarm is not very strong. If I have an important meeting, I use a fallback alarm clock. There are various complaints on fitbit forums.

2) The alarm lasts only 15 secs and it repeats 2 (or 3?) times after 9 minutes. Unfortunately you can't change this behaviour. Moreover, when I'm dizzy I sometimes confirm the alarm and fall back asleep, which happens too easily while lying in bed...

3) other topic, but annoying: the watch is not waterproof

so always keep a fallback alarm. but otherwise it's a great watch.

The Charge HR does exactly that, but doesn't do any effort WRT alarm during light sleep (dunno about Charge 2)