I tend to think Canto Bight suffers more from being the slowest and most convoluted scene from a series of questionable plotlines. As others have pointed out, the impetus for Poe sending the two off on their missions is already muddled by Holdo being underwritten during her assumption of command; I actually don't mind the overall outline of Poe's subplot or Holdo's part in it, but Holdo herself would be written better and Poe's story told better is she offered something besides a late, empty and oxymoronic platitude about hope (seriously, it's a moronic proverb) when anything like "We don't know if they've got spies aboard," or "we have a plan, and we're sticking to it" could have better for her character and highlighted Poe's failures.
Then you've got the reason they're going to Canto Bight and the tech-tech they apparently need and what ends up hurting them; hacking someone's sensors to "cloak" or "decloak" a vessel so you can sneak past shields. All within 18 hours, which apparently is enough time for a significant detour on Canto Bight. Because the First Order has no support ships for fighters, can't hyperspace in front of them and cut them off, and for some reason just scattering the Resistance in transports isn't an option once you get out of range of their turbo lasers. And all when to the audience, it looks like simple visual observation would give the game away for everyone; the Libertine isn't really hiding out when it approaches the Supremacy, and there's no real reason why someone can't just use a telescope to see the transports escaping the Raddus.
All that before we get onto Canto Bight and the actual bulk of Finn and Rose's plotline.
I loved it, but I would have made ONE big change the aliens. How in the world, with our advanced filmmaking technology, do aliens from40 years ago look more realistic than the ones here in the Sequels. And if you can't improve, just reuse the existing designs! One of the best forms of connectivity between the PT and OT and the cartoons was that while you got new species introduced, you were still always seeing the familiar, iconic species established in the OT. Do all of the old aliens just avoid this corner of the galaxy and the Resistance recruitment like the plague because they're scared of the shiny, cheap looking species that inhabit these parts. The only old aliens we see here are the rebels from the OT like Akbar and Nien Nunb!
Personally, I really appreciated the reveal that Finn and Poe's mission didn't ultimately "matter". That's the reality of war. No matter how hard you try, sometimes your best efforts are in vain. What DID matter was the development of Finn as a hero and the establishment of Rose as a major character, which the storyline handled excellently.
Here's where you get into my predominantly subjective arguments against Finn and Rose's plotline, along with some of my suspicions about what needed up on the cutting room floor and what still made the film. There's always a risk of losing audience interest and character progression with a plot-cut-de-sac, especially when the last film featuring Finn had him be the underdog going through incredibly dynamic growth throughout the film, and you've prioritized him 5th or 6th in terms of plot importance. Even the stated intention RJ had with Finn's subplot (that he learns to fight for more than just his friends), is both significantly smaller than his progression in TFA and I'd argue is much less successfully executed, probably because scenes got removed.
Finn's the ex-child soldier who was enslaved as a baby and brainwashed for years before breaking free. The dramatic potential of this guy having to re-enter the First Order's world easily outpaces the "look at the abused animals and the child laborers!" thing Johnson wanted. He could still end up failing his objective utterly, and still have a much better story if they kept the reported scenes aboard the Supremacy involving him connecting with and trying to inspire some Stormtroopers to join him before Phasma kills them. It progresses his character in a visceral and emotionally satisfying way, and would give the "*Rebel* Scum" comeback some energy it lacks here, along with an actual vitriol between Phasma and Finn.
Instead, we got space horse races, a totally needless search for a character DJ was going to replace anyhow, an introduction for some kids with an awesome scene later but who really don't need *this* scene since they're more a general idea than characters, and some lip service from Rose about the rich exploiting the First Order's rise.
Here's what I think could have been done with just the Canto Bight sequence to save time for the Supremacy, and to better use Finn and communicate the idea behind his scene: Finn and Rey are going to free DJ, who's their original target with a more useful backstory as a First Order stationed black marketeer with a way aboard the Supremacy, and is in jail on Canto Bight, so they have to disguise themselves on the planet. While there, they have to avoid Phasma and a First Order group who are accepting the surrender of the planet, where the planet's ruler non-chalantly hands over a group of children to become new Stromtroopers. Finn, initially reluctant not to be on this mission in the first place, sneaks off and frees the kids. Rose then manages to contextualizing how the First Order is doing this on even more planets now. The horse scenes are cut, and the Supremacy scenes are kept.
