Originally Posted by
8000
I was completely surprised by Archer's identity in UBW actually, probably because I started with the anime and without playing Fate route or having full explanations for the mechanics of the world, there was no reason to think servants could be summoned from the future. I did get spoiled to his name being Emiya prematurely, but I could only guess he was just Kiritsugu's ancestor.
Technically it is a plot twist, just not one hard to anticipate reading the VN (or so I say, but to be honest I might've missed it even if I'd started there). It's not a McGuffin though, if anything, you could argue the entire rest of UBW is a McGuffin to support Archer's story (which isn't true either, but you get my point).