The pacing is all over the place with the switching back and forth of so many storylines. I can see why that turned people off. But I don't understand the criticism of it being too 'slow' - I thought there were plenty of action scenes to break up the 'character building' scenes. It didn't feel like a 3-hour movie to me. I thought it went by pretty quick.
What I think this film will also suffer from is people wanting a straight, Hollywood, storyline. Tombstone is one of the most beloved westerns because it has a straight storyline, with memorable scenes and catchy lines, enough humor, and doesn't go very deep. It's the quintessential popcorn Western. Simple, focused, arc.
Costner's Wyatt Earp was darker, went deeper into his life and character development, and had less memorable lines and scenes. It was a completely different feel than Tombstone - and Dances With Wolves for that matter. More raw, less of a straight arc, and doesn't draw you in as much.
Horizon to me feels a lot like Masters of The Air - a VERY WIDE arc with multiple storylines that is more difficult to lay out for a casual audience compared to a single, simple, focused, arc - and suffers from comparisons to previous versions with a more beloved storyline.
To me it's a wider-focused version of 1883. I really enjoyed 1883, and I think the same raw quality is there in Horizon. It's a darker story - which I think is a more accurate telling of what these people went through. It doesn't make for a 'fun ride' summer flick. But it's a LOT to unpack. Whereas 1883 was a more focused storyline spread out over multiple episodes.
I understand Costner wanting this in the theater. But I do think it would have been better to unpack as a mini-series with so many character arcs to unpack.
I really like the cast. Sienna Miller is fantastic. I also feel like Costner went out of his way to show both sides of the settler/native conflicts - where there are good/bad motives on both sides. It feels more true to real life, and complicated. He did this in Dances With Wolves as well, and I see shades of that here. It's just a much bigger scope of a storyline. And again, I think the reality of what these people went through is much closer to something like Horizon or 1883 compared to other past western films.
I will definitely be seeing the next one when it releases in the theater.