Chrono Cross's ending thing is a little obtuse but at the least you should be able to pick up on needing to cast some spells in sequence, even if you didn't figure out what that sequence was.
VP's ending pretty much holds the cake for being really hard to figure out. Since it's like, dependent on EVERYTHING YOU DO.
Anyway, I think it's better to like, not look at necessarily whether you finish a game or not, but rather, how many hours of play time do you get out of a game before you think the price was justified? If you never see the ending of some RPG, but enjoy the half of it you did play, is there any reason to feel GUILTY about not beating it? You still had fun, right?