Mountain bikers run on "finicky" derailleurs over rocks and through wet mud and sand all day. They are unbelievably reliable. With the modern 1x10 or even 1x12 systems, you get enough range to remove the front shifter, reducing maintenance even more, remove almost all sources of jumping chain and allows fitting chain guard with better coverage.

SRAM has a famous video where they lay the bike sideways and jump on the derailleur while shifting, without it bending. Admittedly that model is out of price range for most riders.

Ideally you should give the chain a quick rinse and brush of to prevent grime from accumulating. Sure, the internal gears are a bit easier in that regard, because you don't need to get into the cassette. Otherwise from a reliability standpoint they should take equal beating. The cleaning frequency depends a lot on the oil you use also, wet lube will accumulate more grime than dry.