First of all it's not even a gimmick, second of all why does it matter what some abstract person with no knowledge of the context would think of the film, and also how do you know that's what they would think, and finally how much time has to pass before anyone watching the film becomes aware of what's going on and what Linklater's up to?
If you think the movie isn't great, just say why, don't hide behind this entirely pointless line of thinking. No one's getting "caught up" in Boyhood being shot over 12 years because Boyhood not only can't work, it probably can't exist at all without being shot over 12 years. It's the film's entire backbone, no shit it's not great when someone doesn't take that into consideration.
(Birdman, meanwhile, would work fine if shot/edited conventionally; if there's a gimmicky movie in this year's race, it's this one).