For the most part the engines are holding up very well with good care for most. Thermostats at least the originals fail early, before 50k miles, and they are over $100. The original fan clutches were from a gasoline version and not really suited but there's an aftermarket one that operates at the right temperatures and that
could possibly (hopefully) increase thermostat life
Original fuel filters are a 10 micron paper and are not good long term. A 2 micron filter is a good update be it as a second filter or a replacement for the original.
The alternator has a overrun clutch pulley that fails, mine was replaced at 55k miles. It's available seperatly but can be a bear to remove. The easy route is the whole alternator and that's how the dealer did mine.
Flow control and or EGR valves fail often but a tune can resolve that
dopplegangers have not started to fail yet but some have had turbo problems but that may be because of maintence or driving driving habits.