I don't understand the negative, or even mixed reviews. There was nothing wrong with the third act: it was not "too preachy": it was all about what Brad Bird said he wanted to make - a movie about why optimism for the future had dissipated since the 50's and 60's.
Brad Bird remains a fine action director. It consistently looks spectacular, and has elements of considerable charm. (It's true, it's not a film for really young kids, but then, nor were the Pirates of the Caribbean movies, and no one demands that of all Disney output these days.)
But now, let's get into the political analysis and spoiler territory.
As I have said, it is truly weird the enthusiasm with which some on the Right have taken a cue from the likes of Breitbart to hate the film, sight unseen. (Oh no! - a two hour film mentions global warming as a serious threat to the planet for about 45 seconds of its running time, and it must be condemned. Geez, the Right is truly intellectually enfeebled at this point in history. When is it going to recover ?)
To be fair, though, at least one Right wing site gave it a good review. That attracted this comment:
George Clooney, and the movie is being used by Government Motors to push Smart cars.?
I'm pretty sure I already know what the movie is about. No thanks. I'm just sick to death of Communism.
On the other side of the political fence, the movie has attracted a fair bit of commentary about whether Brad Bird is a crypto libertarian, particularly at Slate. (Reason also noted it could be called a version of Atlas Shrugged for Kids.)
I reckon this Slate article answers this proposition correctly: no, Bird is not a Randian fan, and is clearly a supporter of Democrat politicians. Bird has respect for the innately talented (very clear from Ratatouille and The Incredibles), but his stories also emphasize the talented fitting in to society and benefiting it collectively.
I mean, (honestly, clear plot spoilers about to be stated) even in Tomorrowland, the two characters who espouse the wonders of said titular dimension as a de-regulated realm where the best can succeed free of restraint (a distinctly libertarian idea) turn out to be evil killing robots; and the guy who has decided to keep the rabble out of his version of Galt's Gulch gets killed (we think) in the end.
Aren't those plot points a fair enough hint that Bird thinks talent should be free to have its head, but that's about where his libertarian/Randian sympathies stop?
And a final point: the movie has made me realise that any movie which heavily features rocket packs is likely to be enjoyable. I didn't mind The Rocketeer all those years ago; but even a dark film featuring them, like Minority Report, was also good.
But it must just be a rocket pack, not a rocket suit. (Based on the fact I don't care for the Ironman movies.) My rule of thumb regarding movies with rocket packs is quite specific.