Not an owner, but I've been going through NHTSA's complaint data for the F30/F31/F34 3 Series (2012-2018), so here's what US owners actually filed. There are 1,215 complaints through mid-October 2026, and the engine leads with 29% of them, at a median of 64,000 miles where an odometer was given.
The documented big one is the N20/N26 timing chain guides: the plastic guides embrittle, the chain stretches or jumps, and the engine is often destroyed, typically around 50-80k miles. BMW of North America extended timing chain coverage to 7 years / 70,000 miles, and a US class action covered 2012-2015 cars. Smaller but common: N20 oil filter housing and valve cover gasket leaks, and N55 valve stem seals (blue smoke on startup) on 2012-2015 cars. Electrical is second at 23% of complaints.
Curious whether your yard sees the timing chain failures as often as the complaints suggest.
https://mileagecurve.com/bmw-3-series-2012-2018/ (I run this site)