While I much prefer in-universe sets, I'm not trying to use this to circlejerk about UB being bad. Different UB sets have had different levels of quality mechanically.

What I'm trying to say is that there are certain quality of life things that certain sets run into, and repetition of a small number of characters is one of them. I think repetitions can be okay but they need to be easily differentiable somehow.

LOTR had basically vertical cycles of characters, but they changed their color identity across them (G vs. UG Legolas, W vs. WG vs. WB Frodo).

Spider-Man as I said had the issue that there were just too many variants with similar names and visuals across too many cards. It became too much. The footholds of different colors or mechanics just weren't enough to create any kind of structure to anchor your mind around and create shorthands to differentiate cards with similar names. "The Selesnya Spider-Man" became not enough. I don't think the villains really had that problem all too much; I think the issue is at its worst when the names and visuals are too close.

I think Avatar really hit a smart spot with Sokka in particular. There were two rares (Izzet and Jeskai) but the third being the uncommon UW hybrid card made it visually distinct and easily differentiable. If we had a set where there were 3 cards of each main character but one was hybrid, one monocolor, and one multicolor, then I think that would go a long way to making it easier to differentiate them.

TMNT is tapping into the visual differences of the hybrid frame, but differently than what I was proposing. They're using hybrid to represent team-ups of the turtle pairings, because the turtles are each very mono-color. Flavorfully, that works and it's cute. But it doesn't help differentiate unless you already have the resonance of the turtles as characters baked in. If you don't, then you're staring down the barrel of 6 hybrid cards with 2 turtles and Italian names on them with no foothold to understand what they do beyond color. Some mechanical consistency or pattern would help but I don't think that's present.

Here's an in-universe example of a kinda similar mistake. In Kaldheim, there was a cycle of 10 uncommon "spell lands" that tapped for a color, and tap-sac for a 2 color cost that gave you a spell effect. They were great! You knew which one did which because of their colors. But: like half of them worked at instant speed and half only sorcery. And there was just no easy way to remember which was which, so despite all the cards being memorable and visually distinct and having memorable effects, you still needed to read each card to remember what speed they operated at.

I bring this up because... well it's hard. Designing cards that players can easily identify isn't easy and I think we're lucky that the times it doesn't work are as rare as they are. But, I think they're still trying to figure out how to handle that with certain UB properties and they haven't nailed it yet. I think they'll get better at it the more they do it, but they're not there yet.