I disagree with this, in regards to Finn.
I think people just tend to assume that because he defected from the First Order that he was part of the Resistance at the end of TFA.
He wasn't.
He was on his way out of the system, after Maz's, and only went with Han because Rey had been kidnapped. He lies to the Resistance command and tells them that he knows how to shut off the shields just because he wants to rescue Rey. At this point, he still isn't part of the Resistance and doesn't really care about them, he just wants to save Rey.
At the beginning of TLJ, he is caught by Rose trying to leave, because, once again, he isn't fighting for their cause, he's just worried about Rey.
It's only after seeing all the stuff on Canto Brite that he realizes that he should help the Resistance fight the First Order, and when Phasma calls him "scum," he replies that he is "Rebel Scum," signaling that he has finally realized that their cause is worthwhile and he is finally onboard with it.
This is Finn's character arc for TLJ. Maybe it's not as flashy as everything that Rey and Kylo go through, but it's there and it's important to him as a character. It's setting him up to be a "hero" now.
Yep. Finn's arc in TLJ is deciding he wants to fight for a cause, not just for Rey or his friends.
He (and Rey) both start TLJ with perceptions that everything is black/white and good vs evil. A lot of people think Canto Bight was pointless, but I didn't think so at all. A little long? Sure. But definitely not pointless.
Here's my take on Finn/Rose/Canto,etc.:
The entire subplot is set into motion because of Poe. Poe disobeys orders, which ends up getting a ton of people killed, and essentially strands the Raddus without any real defenses. One of the "plot holes" I keep seeing people complain about is that Holdo doesn't just tell Poe the plan. Think about real life, or even if it were Leia instead of Holdo and a random pilot instead of Poe, why the **** would you tell him your plan? You're brought in to command because some **** just went down, and this random pilot you have never met comes up to you and asks what the plan is (which is really your only chance of survival) after they just disobeyed orders, got people killed, and got demoted. They make it clear that Holdo knows everything Poe just did. There's absolutely no reason that anyone would tell Poe what was going on. If anything he deserved more of a "**** off and wait for orders".
So, Poe, thinking he knows better than everyone - takes it upon himself. When Finn and Rose come along about the hyperspace tracking (which everyone else already knows about), instead of taking any info to Holdo and crew about their idea - Poe decides they should take it upon themselves. So that's big mistake number one.
As far as Rose, I saw her as sort of a character that exists for Finn to gain perspective on not only how others see him, but it occurs to him that people are joining this fight because they believe in it - even if they know their chances are slim.
When he gets to Canto Bight, Finn is initially like "This place is dope, why not just stay here!". Finn soon learns that there are people doing bad things everywhere, not just the FO. But more importantly, he learns that the war isn't black and white. There isn't objectively a good guy and a bad guy. When DJ shows him that the weapons are being sold to both sides, Finn starts to get it. Even then, Finn and Rose trusting DJ is a mistake in itself because they've just seen what all of these people are like, and that they'll do anything for money, but again - they make an assumption that a person is good or bad based on 1 or 2 actions, and misplace their trust (ultimately leading to a betrayal).
Poe messes this up even more later, after learning the "secret plan", he tells Finn/Rose and whoever the codebreaker is on the ship (which again, is a mess up on his part).
The ONLY reason anyone knows about the resistance evacuating to Crait, the entire crait battle happens, etc.. is all because of Poe,Finn, and Rose.
Once Finn is back on Crait, he decides to fight even though he knows the chances are slim (which previously he's been all about running, or just saving Rey). Rey is completely fine at this point. Finn is ready to sacrifice himself because he now believes in a cause, not just his friends or not just because someone told him which side is good or bad. I know a lot of people don't like Rose saving him, but I took it as kind of her learning the same thing that Poe did. When she says "We're not going to win by killing what we hate, but by saving what we love", that to me was basically her saying "hey we need to be defensive and smart because they will easily kill us all otherwise".
Poe learns to play it smart, and not just go blow everything up all the time. So at the end of TLJ, he's more in the position and mindset that Leia wanted him to be in (which he wasn't at the beginning).
I thought every character had a pretty solid arc for the most part. Rey and Finn less obvious than all of the others, primarily because they had the biggest in TFA.
Basically:
Finn - discovers everything isn't black/white and decides he wants to fight for a cause. Stops running from everything.
Rey - also discovers things aren't black/white. Learns more about Kylo/Luke, that her expectations about the jedi and everything aren't all true. She also learns that the dark side doesn't have the answers she wants, and to stand on her own two feet instead of waiting for someone to "show her where she belongs". And obviously she and Ben have a really good arc together imo.
Poe - Goes from hotheaded pilot to being primed to be a leader, knowing when to back off and be smart.
Luke - Arguably one of the bigger arcs in the movie. He's bitter about "the legend of luke skywalker" the entire movie, describing that the Jedi were flawed and that it all needs to end, trying to make it clear to Rey that he's just a man, not a legend. Overtime, with the help of Yoda, he learns to allows himself to fail, and becomes at peace with what has happened. He realizes that the Jedi can continue, because he once again believes that there's good out there. And that the good out weighs the bad. He's no longer afraid to fail, or to let others fail. "No one is ever truly gone". I think that, even though he doesn't try to save Ben in the moment, he knows it's possible - just that he's not the one to do it. So he regains his faith there a bit too. But most importantly, Luke embraces the force and the myth he has become around the galaxy. He realizes that he can do more as a legend (force ghost, etc) than he ever could as just the man. His final act is probably the most badass thing anyone in-universe (or us) have ever seen, and at that moment it's not about power or destroying anything. It's not about destroying the dark side, killing sith, etc like the prequel's flawed jedi. It's about inspiring the galaxy, bringing hope that obviously will have everyone choosing sides, and also inspires all the young force sensitives out there. He's at peace again, and becomes one with the force.
Kylo's arc is incredibly more obvious, and dope as ****. He let's go of the past and stops trying to be like Vader, and in doing so does what Vader always wanted to but never could. However, he also finds that (once again), the dark side and power do not fulfill him. He essentially rules the galaxy, but still feels alone. This is parallel with Rey's scene in the mirror cave - where she finds that no matter what you look for with the dark side, you'll only find yourself. Kylo is kind of opposite Anakin in a lot of ways, but to me it's primarily that Anakin uses the dark side because of his love for others, and then is overcome by his lust for power, losing everything else. Kylo wants power at first, but his love for others makes him conflicted and even when Kylo has all the power (and all that Vader wanted), he's not fulfilled. I think that ultimately, the only person that can "save" Kylo is himself.
Spoiler: 
But, I don't understand why Finn would be motivated to join the Resistance by realizing that there are shades of gray between good and evil. As Ton Ton explains . . . 
