My 11 speed developed a terminal fault after 1000miles, but up until that point it was never truly reliable – skipping, very fussy to minute cable tension adjustments, hence more prone to skipping in a MTB setting.
I sent my slipping 11 speed back under warranty and it had a full service and shipped back to me, it was sort of ok for 4 months but still slipped gears 2 or 3 times a ride when under load going up hill.
Last weekend it wouldn’t stay in 2nd at all.
Did an oil change (oil expensive) and noticed the cable had stretched a fraction, tightened this up. But not ridden it since doing this so I can’t comment.
If I could have my time again I would defiantly avoid any Shimano internal hub and go for a 1 x 10 set up.
Or maybe a Rohloff