I would not let Bes 2016 results deter you from moving to Marshmallow - many people, such as myself, have upgraded to Marshmallow with absolutely no problems.
Nexus 5X on AT&T - Posted via the Android Central App
What device? Just because you're on T-Mobile's network does not mean that your device will follow T-Mobile's update schedule. You're on an MVNO, you could have an AT&T device, which would not follow the same schedule.