I want to love Shimano twelve speed, but I went XT/SLX mix a little over a year ago and I am dissapointed.

-Blew an XT chain, out of the blue, in two places in the middle of a regular chunk of trail. About as JRA as you can get, and the chain disintegrated. It was ~a month old

-SLX chainring failed when a couple of those anti tamper bolts backed out, and the chainring bent in that area under power. Only Shimano could design a direct mount chainring with 8 bolts... Warranty was XTR, which is never worth the premium for me, but at least is a one piece part, as all direct mount rings should be (quick change,  CAMO/Switch aside). Failed a couple months after the chain.

-XT shifter stopped down shifting - lever just swung freely. Warranty took 4 months. It was ~3 months old.

- 9 months later, warranty XT shifter does the same thing. Warranty only took a few weeks.

- Two days ago, grenaded a year old, 2000 mile chain. Chain checker showed it about half life. 

I've also found that XT 12s chains rust, even in my dry midwestern US clime. I don't have that issue with any other chain in the fleet - some cheapy KMC, 11s XT, and a variety of chunky single speed chains.