Shit like "we did chain driven timing and it worked, but in new engine it prematurely wears", or "someone decided it is fine for belt-driven timing to take dip in oil, oh surprise, we now have parts of the belt landing in various parts of the engine".

Or my favourite, "metal, belt driven water pump is almost never replaced thru the lifetime of the engine and it's cheap, let's make propellers plastic and also drive it from separate electric engine"