OP: It's simply impossible, obviously, for a complex system to be completely clear and absolutely independent of interpretation. That, of course, doesn't mean that such an impossible state isn't desirable, and that one shouldn't go for it, and it doesn't mean that rules systems that fail to cover the basic things they should cover aren't bad systems.
Now that's not entirely fair. Some systems do, genuinely, have fewer numbers of rules, it's just generally to the system's detriment.
Take Apocalypse World and its ilk, for example. In it, there really is only one rule, which is suck up to the GM to hope they don't shaft you, because it is literally impossible to generate a meaningful result from any task resolution in the book. And, no, I'm not even talking about how the GM decides how your roll is resolved, I'm talking about how, in text examples in the book, there are explicit examples of a "success at a cost" on a hide attempt wherein the cost was "you fail to hide," and examples where a successful check simply means you fail the mission. There are no rules beyond sucking up to the GM, because any actions the players attempt to take are entirely meaningless.
On the other hand, take Munchhausen. Despite not really being a huge fan, it really is the only rules-light system I would argue is, in fact, a good game. In it, there is essentially no vagueness, and the rules can be completely explained to even a complete newbie in under five minutes, and it is both conceptually and mechanically sound.