In general I judge a story based of the best character 90% of the time but here it's kinda undeniable for almost anyone, as the game doesn't offer much other than Arthur (there's not much story other than him, the cast is weak and the gameplay is just not very good imo, and even if the cast was good Arthur is the best of them), so i'll just go over his characer.

The problem is that Arthur really is just not that good. He suffers from the rockstar character writing that was very present in gta 5.
Arthur's development of redemption revolves, or should revolve around a dynamic that is just not good, to the point where it's hard to even see how this dynamic made him change, and i'm still not 100% sure it did lol.
We barely know how Arthur got to the point he's committing every shitty thing Dutch will tell him to just because he tells him to, as we never got to see the past, only were told of it in like 2 sentences. So we don't even know what kind of man Arthur was before either, how Dutch had changed him, what Dutch taught him or why he's so loyal to him despite Dutch betraying him so many times other than that we should just accept that as the reality of their relationship because the game started with it being in that point, because other than that literally all we know is that Arthur's parents died and that Dutch raised him, that's it.
The other problem with this dynamic is that not only does Arthur never truly leave Dutch and develops beyond that (he always seeks his validation, even in the last scene of the game), his reasoning for this, I guess, beginning of what could have been a development, him starting to doubt Dutch, only come from Dutch betraying him many times or Dutch becoming mad, it doesn't come from Arthur's own understanding, but only from Dutch's own change, he doesn't even reflect upon his relationship with Dutch or anything (MAYBE one line when he was talking to John at some point that doesn't go in depth on it at all). He just realizes that Dutch went too far which helps him start acting on his own will instead of what Dutch had taught him. For something that should be the main fuel for him to change for the better, it's just very poorly done imo .

Same thing happens with Arthur's feelings before the development itself, we are only told that deep down he's a good guy by other characters, it has no real substance. I guess it happens because we as the player have high honor cuz we did some good stuff, but it just doesn't add anything to him, the way it feels is that it's not his decision but ours, we don't get insight to why he suddenly helps people as it's out of character, one moment he doesn't give a shit about murdering innocents and beating up people, and a moment after he (or we) helps a homeless guy. Even if it was part of the main story, to me it just doesn't make any sense, he's still a horrible guy and his kind acts would make no sense whatsoever, it's uncharacteristic especially when not given his inner thought on why he does that or that he wants to change for the better or anything like that (just look at Bojack as an example of how it should be done ig). It literally makes much more sense for his character to play him with low honor (which I did not btw), it's just a mechanic the game has for us, not really for him. So him being a "good guy" isn't really a thing that makes sense, nor does his regret make much sense because all we see from him is committing horrible stuff, beating a guy to death cuz he didn't pay his debt and feeling nothing about it. IIRC there's even a scene where after being told that he's gonna go to hell or something like that for what he's doing, he himself even acknowledges how bad of a person he is, but that he just doesn't care, he says something along the lines of "it's part of life" (don't quote me on it i'm sure it's not exactly that but something similar).

So everything i've talked about so far is basically his reasons to develop, why he should develop, what makes him want to develop, which is such an essential part to character development. So when we get to his development, if we simplify it, it's basically this:
Arthur, horrible man, who learned to be the way he is because of Dutch, who never had any remorse or care about any horrible shit he's done, suddenly regrets everything cuz he got sick and because he realized Dutch isn't that great of a guy so he should start thinking for himself instead of going after what he taught him. (which btw he has no reason to suddenly become a kind man when he thinks for himself, as we again, never saw what Arthur is when he does think for himself, which is also another reason for why when developing a character towards something like this, you should make the dynamic push the character towards positive change, not just push it away from negative change)

The funny thing is, even if everything i've talked about so far was done better (Arthur showing he's more than just a merciless killer before getting sick, that he has regret and empathy, or having more of his past with Dutch shown, what this relationship means to him what Dutch taught him, why he's so loyal to him, just their dynamic having more depth in general + even if Arthur did actually fully detach from Dutch and go on his own way without carrying about Dutch or his loyalty to him at all, which again isn't something his character went full on with) It still wouldn't have been good. His development is just fundamentally not that good (maybe would have given it a 3.5 then), the whole way it's done is not the way a powerful development in my eyes is done. Again, with Dutch, his only reason to start developing is him getting external invalidation to his loyalty, nothing more than that. He is very loyal to Dutch, it turns on him, so he understands that following Dutch is bad. Same thing with how he develops into doing good stuff, he did a lot of bad shit, he gets sick and Dutch became mad, it unnaturally (it being unnatural is basically what I explained so far, the point now is that it is not that good regardless of it being unnatural or not) pushes him to regret everything, so again same thing just with invalidation that now comes from his own feelings (that again don't make much sense considering the kind of character he was portrayed to be so far), everything falls on him he understands being a bad person made him feel bad so he decides to become a good person. In my eyes, for this example, a character learning that it should become a good person because being a good person is good rather than that it should become good because being bad made it feel bad so it runs away from that and goes to the other direction, is infinitely better, which Arthur doesn't do. It also is a lot more logical, makes more sense, feels more natural and makes for a much stronger message.

Now we get to chapter 6, where Arthur spends most time showing his development, he helps people and they keep telling him how amazing of a person he is even though moments ago he was the biggest piece of shit ever etc etc.
Arthur himself doesn't feel like he is a good person worthy of those compliments, he keeps saying "you don't know me" and stuff like that, but after getting enough compliments at the end of the game he remembers all the times people told him he's a good person and so he decides he is and dies in peace. This is again, very weak, it's external validation (not good, pretty easy to understand why), people told him he's a good person so he just accepted that as a fact, he clearly never felt this way himself. His development just doesn't work for me.

And don't get me wrong, I can totally understand why people do like him, this game was in my top 5 and he was one of my favorites for like 2 years. It just grew off me, mainly because I developed different values to what I used to have, much of the stuff I talked about here most people don't even really care about tbh, which is fine.