For an animated kids series that showed surprising maturity and self awareness, I never expected it to have such a bad ending. While I appreciate all the set up and some of the pay offs, most of them come in convenient and lazy ways. The show is actually quite contrived most of the times, but I always forgave it since it's a kids show, but it goes too far in the finale.
Bill is literally an all powerful being that can control reality, it's already contrived that these weaknesses like the weirdness magnetic force in gravity falls, unicorn hair, or his weakness exist, but at least that's set up. What is contrived is how all these characters happened to be found, they all work together, build this giant robot, fight the worst inter dimensional criminal monsters, even fight the all powerful Bill (he really didn't notice the arms and legs were outside the force field?! That's like the first thing you'd notice) and send the perfect people to his lair, with the perfect amount of time to set up Bill's weakness when Bill could literally just kill them all in seconds, and then when the show should've ended, Stan puts everything at risk for grammar... Seriously?
Somehow the final episode manages to be worse with the biggest fairy tale ending where everything goes right. I actually quite liked the twist with Stanley and Stanford switching. I actually thought it would be the clever way to end it, and it would give stakes to have Stanley basically "die" (well just forget everything), but then he soon remembers everything. Bill is defeated and everything reverts perfectly back to normal, and every character gets the fairy tale ending. Stanley and Stanford get their adventure with the Stan O' War 2, Mabel and Dipper stay together (which completely ruins Mabel's arc) getting their birthday even with Mabel's friends who were supposedly not going to make it, Soos gets to be the owner of the Mystery Shack, and when everything is literally better off for everyone (except for the Northwest's, which hilariously the parents are the only ones who developed at the end) Mabel even gets to keep Waddles. They make big deals out of every moment even though it's obvious it's going to end perfectly for everyone. For a show so focused on its continuity it actually leaves quite a few things unexplained, stuff that it pulls out of thin air, and far too many conveniently tied up character arcs (or actually lack thereof) or plot lines. They really got rid of the government that easily?
It's extremely disappointing because the first 2 parts seemed quite good. I liked things like the changes of the intro, the seemingly darker tone, the apocalypse, dream vs reality, and some other stuff, but at the same time it doesn't really hold up thinking back at it. There's multiple of these eye things and people like Soos just appear perfectly when the characters need him? Or how Gideon is seriously turned by a speech? Again the entire show is quite contrived, but most of the 4 part finale relies on pure convenience. The penultimate episode before the 4 part finale was one of the best too, setting up all these great things like Dipper moving on and leaving Mabel, high school not being the fantasy she wanted, her birthday being ruined, summer ending, and the set up of the weirdmageddon, with the heavy hitting themes of growing up/moving on. But nope, now we just get all the characters fighting in a big robot, with the characters regressing, having fun, and getting basically everything they wanted in a fairy tale ending. Sorry, but this is an awful ending, and again I would've been much harsher if it wasn't for this being a kids show and the rest of the season being so great.