Let's talk about metaplot for a moment.
Metaplot, in RPG terms, is the overarching narrative of a game line. That there's going to be a connecting story through all the adventures and supplements of a game, usually regardless of the actions of the players in individual campaigns. You don't see them much nowadays, because they're a bit of a codependent concept to another very 90's idea (the supplement treadmill), but the main reason they've fallen out of favor is because a metaplot ends up meaning that you have to play your campaign the way the designers want you to, instead of how you want to do it.
It wasn't uncommon at the time to be presented with major villain NPCs that the players would be expected to constantly butt up against, but were unable to defeat due to that NPC being needed to trigger a major plot point down the road. Or for one book to have a major plot point hinge on a factoid from a different book you might not own or be able to get. Or having certain character types invalidated because of changes to said group that happened in one adventure, with every book released after that working off that idea.
(The most infamous version of that last one would probably be when White Wolf killed off the entire Ravnos clan in one book the oWoD, meaning that if you were playing a Ravnos you were either one of the last ones left on the planet and were pretty much on your own from then on out, or dead due to metaplot.)
When you're dealing with metaplot that goes through a game line, you (the GM) have a problem. See, a metaplot means that a publisher has a story they're going to tell, and your group is going to be going along for the ride.
Now don't get me wrong, I'm all for story in RPGs. Hell, stories are the point of RPGs. But what I'm talking about isn't emergent storytelling, it's about railroading. To wit, it's about having to bend your campaign around the tidbits revealed piecemeal through supplements instead of being able to pick and choose, and the story you're telling with a game not the one you, the GM, wants to tell, but the one the game writers want you to tell.
For example, let's say that you're playing a modern-day campaign set in Boston. You're setting your campaign there not because it's the default city for the game in question, but because it's where your group happens to live. But then in one published adventure, Boston gets nuked, and from that point on every supplement works off the assumption that Boston is gone, period. So now you have two general options:
1) You can play that as it lays. Now you have to rework your campaign around the fact that everything you'd established in the campaign is gone, forcing you to pretty much start over.
2) Ignore it, and keep playing the campaign as you were before this happened, meaning that any official supplements that came out after that were either useless or would require work on your part to shoehorn them into your campaign, which now deviated from the core game line assumptions.
Now, clearly I'm oversimplifying here, but you get my point: games with heavy metaplot are a real pain to deal with as a GM and as players.