Are you a developer as well?

How exactly is it unstable?

Hmm .... From my own personal experience, apps are fine. Maybe I don't use as many as you do, but with the recent updates, many apps have already been made stable. I use Whatsapp most, which works pretty much at a satisfactory level now. Maybe you can tell us what kind of apps?

Also, if you're using a Nokia phone, its apps are superb.

How is the OS unstable? Lack of some features here and there, yes, unstable? Ever since GDR1, my 920 has never had a random reboot, it doesn't lag ... How else is it unstable?

The battery app cannot be running all the time. For Live Tiles, the battery app can only update itself every 30 minutes hence that's why you notice a sudden drop / same level phenomenon. The most accurate way to gauge your battery level is to open the battery app (and keep it open), but that's hardly necessary, unless it really bothers you that much. Most battery apps come with graphs, while can't be 100% accurate, can provide you some insight into your battery performance. Remember, Live Tiles can only ever update every 30 mins (for time based events).

Then that could be battery's problem ... But like the old saying goes, if it ain't broken, don't fix it