There is a flaw in the elevated stay design on the Stache. In SS mode there is just enough flex that it allows the chain to pop when under power. Mostly only riders over 200lbs are experiencing it.
I know several guys (some are active mtbr members) who are running narrow/wide chain ring, narrow/wide cog, and tighter than normal chain tension to try to avoid it. Seems to fix it most of the time, but it still appears in rough terrain. Have heard of some geared riders experiencing it as well.
Trek is aware of it and openly admits to the issue, they state it was never intended to be ran SS. The sliders are only there to allow change between 27.5+, 29, and 29+ wheel sizes.
Some people are willing to overlook the issue, some are not. I was going to buy a Stache for my main SS ride, but have decided against it for this very reason.
Not trying to bash Trek at all, it's a very fun bike. Just trying to make sure you're aware before you purchase.