Reliance on non-repairable electronics everywhere, fully sealed mechanical parts, etc. Efficiency comes at the cost of more moving parts, more sensors, higher pressure, tighter tolerances, which all introduces more failure points harder to debug/replace. Not to mention lighter thinner non-cost prohibitive materials are easier to damage.

As a rule of thumb, I will only buy pre-2008 vehicles, things only went from bad to worst after that era.

And despite the additional complexity, reliability has improved. I have an old carbureted vehicle, and while it might be simple, the environmental shortcomings it has well outweigh any o2 sensor that might need replaced at 200k miles.