I had to replace my EX1 derailleur after less than 300 miles - it literally snapped on a moderate climb, taking a spoke out and bending a chain link with it. It was working fine at the time, and I'm pretty sure it didn't hit a rock or anything. Bosch CX w/shift detection - but I still ease off when shifting. I really don't think I was at fault...I don't know about anyone else's experience with the EX1 groupset but mine has been mixed. At first it changed smoothly even under pressure - I'm a big guy and put a lot of power through the chain. It started to jump and I changed the chain before it really needed it. With a new chain on - 200 miles later and the rear cassette is now so worn it jumps regularly out of gear.
In total I have covered 1200 miles in about 8 months. I don't know how long a cassette on a regular MTB would last for? TBH I had hoped for longer.
I have decided to replace the whole groupset for 11 speed XTR. I think the lack of mid-range gears of the EX1 means you end up wearing them more - especially if you spin a lot.
Be interesting to hear how others are getting on with it.
EX1 chain life: I've seen .5% stretch twice now in 800+ miles. My two XT 11-speed eMTBs have generally reached that point every 200 miles or so, and both their original derailleurs still work great.
XG-899 cassette looks and works great. I prefer its shifting and gear spread to my XT bikes.