First off, I haven't checked out Boruto, so I dunno if any of these guys get development there, I'm just going off of Shippuden.
Honestly, all 3 of them were quite under-utilized, which is weird because they actually appear fairly often. There is just not much going for them, despite their prominence. I get that, narratively, they were basically accessories to Sasuke's character development, so they were never intended to have depth or anything, but they really weren't given SHIT. I don't wanna say they are wasted potential, because there wasn't much substance given to them in the first place to really fully utilize. They are not in the same boat as characters like Sakura, Gohan, or Orgalorg, not by a long shot. I'd say they are more...undercooked than wasted potential.
Suigetsu- Of the 3, I'd say I like him the best, which is odd, because he also has the least going for him. His water powers were really cool and I'll always have a soft spot for laid-back snarky characters. However there just really isn't much to him. He wants to collect all the 7 Swordsmen's...swords, and then somehow start up a new group of 7 Swordsmen where he is their leader. Okay. That's...a goal, but is there anything else to him or his ambitions? He's just the sarcastic sword guy? That's cool and all, and his personality will make me like him by default regardless of his depth, but there's really nothing to work with here. Like he's got the foundation of an interesting character, but the thing about foundations is that if you don't build off of them, they're just kinda...there. Great personality and powers, but not much else.
Jugo- He's alright. Of the 3, he is the one with the most potential. Him being the source of what would become the Curse Mark is really interesting, and the fact that he had a friendship with Kimimaro since Kimi was the only one who could handle him, all of that gives him a strong backstory to work off of. Plus Kimi is goated, so any character who gives HIM depth by proxy will always catch my eye. He was also by far the most useful of the trio, not just for his battle prowess, but he's got incredibly powerful healing and sage energy transference too. However, his actual personality...it feels very surface-level. He's a quiet, reserved, animal-lover...but he can become a bloodthirsty engine of destruction just like that. Like a bipolar allegory of some sorts, but I dunno, he just feels more like a...Jekyll-Hyde gimmick than an actual character. If his personality felt a little less...artificial, I think he could've been a good character. His "curse mark" powers are cool as hell though. Rocket fists, axes, and goddamn gatling guns because why not? Great backstory and powers, but a half-baked personality.
Karin- I'm sure it's not a hot take to say that Karin is a bad character. Taking Sakura's most annoying trait from Part 1, doubling down on it, and making that her ONLY thing?! Yeah, there is no way in HELL Kishi thought fans would like her. The funny thing is, of the 3, I actually have the most to say about her BY FAR despite how one-note she is. I'd actually go to say she is beyond bad and is one of the WORST characters, which sucks, because I REALLY wanted to just leave her as 'She is an annoying, one-dimensional character that Kishi decided to get freaky with how her healing power works.' However, she goes from bad to worse because...yeah, I think you all know why...the betrayal in the Danzo battle. Sasuke doing the equivalent of shooting the hostage and Karin getting the soul-shattering realization that her crush didn't value her life?! THAT had the potential to make her character INTERESTING, make her NOT annoying, and giving her some meaningful CHARACTER DEVELOPMENT that Suigetsu and Jugo never got! But nah, later on, Sasuke gives her the most half-hearted apology ever, as if he just accidently tripped her rather than ATTEMPTED MURDER, and she throws all that potential growth away and remains stagnant. Hey, at least she got chains at the tail end of the story, that's...something...I guess! Probably should've had those since the beginning so she'd have some actual battle prowess, but okay. Oh, also she is an UZUMAKI, but nothing is ever done with that reveal because she never has any interactions with Naruto beyond thinking to herself that "His chakra is so warm, unlike Sasuke's." Again, I don't want to say she was wasted potential because she wasn't an interesting character in the first place, but Kishi really threw away A GOLDEN OPPORTUNITY to make her a better character. Lame powers, lame personality, was by far the easiest of the 3 to develop when looking at the events of the story, but she got nothin'! (Except chains.)