I don’t think the FSI engine is particularly problematic, but given that it’s a gen 1 direct injection engine, it does suffer from the known problems that come with such systems. It also has the rear timing chain setup which whilst on the A8 isn’t known to be especially problematic, at high miles it will start to become an issue (everything wears out), and it’s a massive engine out deal to fix with expensive parts.

For ultimate longevity and reliability, the timing belt engine is pretty much bulletproof given it uses tech that is as refined and developed as it got before the industry switched over to direct injection systems. Timing belt service needs to be done periodically to realize that longevity, but it’s not the huge deal some folks make it out to be. And it also lumps in the replacement of a bunch of other common wear items like the water pump, thermostat, rollers, tensioners etc that go untouched on chain motors until they fail and then it’s $$$$$.