Swapping a remote battery or resetting the travel limits is something most owners can do themselves, and we'd rather you save the service call for that. Where it stops making sense is anything involving the motor housing, the logic board, or the drive mechanism itself. Openers carry spring-loaded components and line voltage in the same small box, and getting it wrong doesn't just cost you a part - it can put a door down on something it shouldn't.
The other honest answer: if your opener is original to a house built in the late 1980s, a repair might buy you a year or two, not a decade. We'll tell you straight when a part is cheap and easy versus when you're chasing a unit that's simply done.