A very important inquiry that I have oft thought about.
For one start, I would generally prefer that anything which is complex would be limited to a single game. And if it must expand past a game, staying within one unit of the series is also important.
It needs to be able to be found by someone looking for it. So as I mentioned in that linked question, I am concerned about addressing someone who only cares about, say, Megaman 2 and wants to know a good boss order. It also encourages a better approach by multiple people suggesting different answers when you are only covering one game. Otherwise, you have to deal with trying to cover all of the games in the scope, or whether you only cover some, how you deal with when you have strategies for one game in one answer and strategies for two games in another... it's messy.
Now in general scenarios...
If one person does compile a huge list of all 17,750,000 weapons and can fit it comfortably in one good answer, what reason to we have to block it?
Remember that a good question will be something that is useful, that will attract upvotes. So even if one could list all 17 million weapons of Borderlands, is having it all in one answer actually going to be useful to someone?
We can't really make an arbitrary judgement on a concrete scope because we would end up denying very valuable information that ends up just over that scope. So we have to make due with a subjective analysis when we're past isolating it to a game (or series). The essence: how useful will it be in a broad scope? Is it necessary to compile all of this data in one post? Will anyone be looking for it in this list? Is the huge list going to be more useful than having separate items for when the actual important components come up? Will a huge list cause interference when people need specific strategies and we start closing as duplicates?
Scope is necessary, but it's basically a judgment call on the question, I say. If you have an overly broad question, just stop and think to yourself how important and useful that information will be.