tl;dr: Good metaplot is good, bad metaplot is bad.
We can all cite examples of both. A good author should read and respect the background, and -if their 'brilliant ideas' clash horribly, they should throw them away and write something else, because one ego is less important than an entire setting. Many great plots and settings have fallen foul of discontinuity or a clear clash of opinions between authors (ie the whole 'Clan X is uber this week, because they're my favourites' thing in WoD).
The thing is.... we can always choose to ignore bad metaplot, and carry on playing our OWoD game, pretend Kerinsky was just a merc from the periphery, or whatever. Whereas good metaplot is genuinely staggering and a great asset to the game.
Too much of a reveal to early about plotlines can really spoil things and hamper creativity, and too vague a hint can be useless to a GM. "Well gee; there are ghosts in that place. Thanks. I could never thought of that. Maybe some clues about WHY or the history would be nice, too?"
I don't want 'GM material' that is just paragraph after paragraph of isolated mysteries. I'd like to see things that are fully fleshed out. Maybe the answers aren't all there, or maybe there's a 'choice' of three ideas behind each mystery (as per newer WoD material), but I don't want something that's too Twin Peaks: All questions and no goddam closure.
When the newer WFRP and 40kRPG stuff started to trickle out, my reaction to published plot and scenarios was immediately 'I hope it's as good as the original WFRP stuff'. You remember those adventures? They were some of the finest plots ever written. If FFG can come out with material like that: Great. If they try and it fails... I won't use it. Simples!! I think it's worth the risk of them trying. We might get junk, or we might get another 'Masks of Neothotep/The Enemy Within'.
Oh yeah: Another problem with metaplot is that sometimes it moves too fast. L5R for example is based on a card game, and the metaplot used to tie in to major tournaments. And they always had world-shattering themes. So the RPG was dragged along, and every 3 years there was a massive conflict which killed 80% of samurai and nearly ended the Empire. It wasn't great.
Kain McDogal said:
(they were clearly available at the time of writing, the opposite assumption was often used as an argument why they didn't made it into MotX) and probably would have printed all new weapon tables at least for the Xenos!
I think your opinion on the speed that books go from the writing stage to coming out in print is rather optimistic. Occam's razor would point to coincidence.