I think it stems from the way the character is a fish out of water and how that informed Boyega's portrayal of the character. Mace and Lando were competent and badass. I think Finn is shown to be quite competent but he's not a slick badass. He comports himself well with a blaster and with a lightsaber when called upon, he's a good starship gunner and his knowledge of imperial workings helps formulate the rebel's plans.
It is interesting to see how the character is viewed from different perspectives than my own. They could've swapped Finn and Poe but I think Finn is a way more interesting, more human person who actually has an arc where he grows from a panicked deserter pining for the first girl he's known to a resistance fighter fighting for a cause. Poe is basically just always Poe though he's both badass and competent. It's actually a side you don't get to see from a lot of black sci-fi characters who tend to always be stoic and cool though I don't want to seem oblivious to the criticisms and where they're stemming from.
I do see the sanitation thing is something that could have been left behind.
I agree with you that it's very interesting to see how different people view Finn or Star Wars in general. As a black genre fan, I've long contended with lackluster (at best often) depictions of black characters in many of the stories that I otherwise love or find interesting. Where you might see stoicism I often see the creators/writers not really having any idea who these characters are or what they want to do with them, but yet feeling a need (or activist/ studio pressure) to be inclusive. So they
got to include a black, or black characters, but then they usually stop there, and sometimes celebrate just that step. But making, or leaving black characters, as the 'cool', sometimes silent, noble warrior (savage), supporting character is an easy way to go (and honestly one I find more acceptable than out of breath, panicky Finn). It's not always patently offensive, but it also winds up putting the character into a corner. Other times we get the comic relief character, either a jokester or the butt of jokes, and now I'm seeing more of the black nerd kind of character. Of course there's long been criminals. And we also get blacks in the background, generally mute, but they do add splashes of color and the illusion of a more inclusive, progressive envisioned world.
These are different takes, but mostly at the end of the day almost none of these characters have agency, have interior lives, almost all live (some willingly giving their lives) for the white main characters. Almost all the black males are sexless or we don't get to see them having sexual relationships onscreen. If they are lucky, they might have a wife like Stargate SG-1's Teal'c off planet. Even LeVar Burton has pointed out how TNG made Geordi an eunuch. While some fans identified with Geordi being socially awkward, I noticed even when I was a teenager that this character doesn't have a love life, and I saw that almost every other TNG character did, and it got my attention. Geordi's love life was haphazard at best, and came as an afterthought to me, which is generally how I feel black characters are often treated, even when it comes at the hands of generally progressive writers who don't seemingly harbor any intention malice or trenchant racial bias. It just 'happens' that these characters are sidelined or underdeveloped, or poorly developed.
Let me say that there's nothing necessarily wrong with depicting sexless or socially awkward black male characters in genre works, or fiction in general. They do exist in the real world, however, I'm looking at it from the point of view that if you're only getting x amount of black characters and then even smaller numbers that get some development, then I want to see more fully realized characterization. I want to see these characters experience the ups and downs of love, sex, romance, just like other characters. I want to see them be heroes, and sometimes villains as well (albeit not stereotypical), and we get to see the world/universe hang on the decisions they make. Further, I want to see how the black experience informs those characters' world views and actions and that we see value in African/African Diaspora cultures and experiences more onscreen.
And if almost all the black characters (socially awkward or super cool) are still having the same 'problems' when it comes to love, romance, sex, then there's something else going on there. There's a blindspot or reluctance to make these characters three-dimensional. Inclusion or representation (whatever that might mean) are about all Hollywood seems able to do, or care about. Getting in the door is the first step, a seat at the table is the next, and now it's time to hear these characters speak and learn more about their stories.
And it's nothing against the actors, including Boyega in Star Wars. Whatever positive energy is there from Finn I credit that more to Boyega than either Abrams or Johnson. I was looking at some of The Last Jedi last night (a glutton for punishment I suppose) and I had forgotten that it was Finn's idea to go to the Star Destroyer, which led to the Canto Bight side quest. Granted this all ended in failure and was basically pointless, and he got led around by Rose, but still it did show Finn using some of his First Order knowledge. I wish this aspect to the character had been better depicted and fleshed out. Also it made no sense for the Resistance to embrace Finn as easily as they did. I wish we had seen more ambivalence and hostility toward the character, which could've led to some nice dramatic moments. As it stands, Finn was motivated to run away in Force Awakens and at the start of The Last Jedi. The only reason he hung around was for Rey, at the time for inexplicable reasons. Abrams/Disney even messed up with the idea that Rey is the first girl Finn's known. Females (like Jannah) were First Order stormtroopers. We also see females on the bridge of First Order ships and Phasma was his commander, so he would've seen women before. Further, the idea of conditioning didn't prevent the First Order stormtroopers from showing comical sides or bits of personality, like in Force Awakens or Rise of Skywalker. So I can't say that their conditioning wiped out, or wiped out completely human nature. The fact that they had to be reconditioned from time to time, that could have been one of the reasons for it, the biological drive breaking through.
I think Finn's character arc became more interesting during The Last Jedi when he started to shed his fear of the First Order. He defeated personal nemesis Phasma (with some help) but really when he was ready to sacrifice himself to save the Resistance on Crait (which would've been a satisfying finish to his arc for me, but that was ruined by Rose's save, and more Johnson subversion). Abrams throws in the Force sensitive thing, but of course there's no time to develop it further, or to build on his relationship with Jannah and the other First Order deserters. And Boyega has said he's not going to do a Disney Plus show. He might change his mind in the future though, and though it seems both he and Daisy Ridley are done with Star Wars, who knows if they will feel that way in a few years, and they might be back for Episode 10.
Overall, just about all of the sequel trilogy characters were poorly developed, not just Finn. I think Disney was banking on the strength of Star Wars as a brand to bring in the older, and presumably, whiter fan base while diversifying the lead roles would make the franchise more appealing to diverse audiences. I don't think that's necessarily a bad idea, but Disney forgot to actually make interesting characters.
I do think Dameron had an arc. It wasn't there in Force Awakens, but he had to learn how to be a leader in both The Last Jedi and the Rise of Skywalker. I do think he was turned into a hotheaded quasi-sexist jerk though in The Last Jedi to give him this arc, and then he was made into a spice (drug) runner in Rise of Skywalker to also give him a backstory (perhaps to make him more like Han Solo). To me, it's just goes to show how the sequel was not planned out well, and they were making things up as they went along. So, Finn could be bumbling one moment, but then an ace shot the next, etc. Rey was looking for identity in Force Awakens, told it didn't matter in The Last Jedi, but then that it did again in The Rise of Skywalker. We still don't quite get who she is, or what she wants to do, but at least Abrams finally gave her a personal stake in the story, even if her struggle was reminiscent of Luke and Vader. I wish we had gotten Rey talking more with Palpatine to see if she could 'turn' him. Granted he's the ultimate evil, but he was still her grandfather, and I wish we had gotten more between them, but the film was so rushed there was no time to flesh it out. I wish Disney had just made an Episode 10 at the same time they were filming Rise of Skywalker, end Episode 9 on a massive cliffhanger, and then have at the end of the film, let us know there was an Episode 10 coming in May or something.