macOS Sierra, Version 10.12.4 (16E195), MacBook Pro (Retina, 13-inch, Late 2012), Magic Mouse 2.
I know you can see the current battery level in (Apple Menu->)System Preferences->Mouse.
But there is no more easily available indicator, f.ex. in the Menu Bar, and more importantly; there is no Low Battery Level warning of any kind (the system should e.g. give a notification when the level drops below a configured level, and remind you to charge).
The lack of these simple and obvious features just made my mouse stop working without me having any idea why, in the morning of my work day, so I had to find another one at the office. Could have been avoided if I had left it for charging last night.