If there's one thing that the MCU has managed to do better than nearly anyone else, it's their brand of having a giant connected universe. Most of their films connect back to each other and feature characters that overlap between stories. Overarching plot lines or villains - like Thanos - are set up years in advance and don't pay off until much later. It's a really interesting way of telling stories because each movie exists within a larger ecosystem where the viewer probably needs the context of 5 other previous movies in order to fully appreciate the newest one.
Of course, this is also the biggest drawback and flaw to this system. Every movie in the MCU is starting to feel the same because they all need to feel like they fit within the same universe. Characters are often shoehorned into various stories just to create a crossover or add a bit of shock value to a post-credits scene. In trying to create a cohesive universe, the MCU may have gone too far in that direction and made it so that every movie feels less and less like its own thing. The problem with trying to connect all the movies in one franchise is that they lose originality and start to feel more like poorly executed fan service than anything else.
It's rare, and actually pretty much non-existent, for an MCU project to release that's just a singular story. Very few Marvel movies in recent years can exist fully on their own with no prior knowledge of the universe. This might be fine in some franchises with fewer movies, but if a person needs to watch 13 years of movies in order to understand the references in the newest one, perhaps it's a bit of a flawed system. At this point, it just feels like Marvel is making movies that are only for long-time fans, which in a way is nice because it shows that they care about their fans, but it feels like they're relying too heavily on their notoriety and not actually putting in any effort to bring in new fans.
The creators behind these movies are also not really able to use the full range of their creative talents or bring much of their own style or flavor to the MCU films they work on. Again, all of the movies have to fit a certain style and aesthetic to the extent that they all feel like they're from the same universe, and this limits what the directors or writers can do with style. Taika Waititi has come the closest to creating an MCU movie that feels like it really came from his hand and has his distinct style throughout with Thor: Ragnarok.
Eternals came out right after Chloé Zhao had won the Oscar for Best Director for her film Nomadland, which has a very striking and unique visual style. There are hints of that style in Eternals, but the visuals still feel very much like a Marvel movie, which is to say, uninteresting. Zhao's touch is more evident in the storyline, as it's a very mature and complex conflict. However, though this was somewhat unique for Marvel, it didn't go over well with a lot of fans, which is only cementing in minds of Marvel executives that the formula should never be changed because the fans who are comfortable with that formula seemingly hate when it's challenged.
This is a problem because it's so limiting to the potential Marvel has to make really cool and unique films. It is rumored that the upcoming Doctor Strange in the Multiverse of Madness will lean more towards the horror genre, especially with Sam Raimi as the director. However, if anyone is putting their hopes on it being Marvel's first horror movie, they should temper that expectation. Even if it has horror elements, the MCU would never fully lean into that because it would be too far outside of their norm, at least at the moment. It just seems like they're afraid to take risks because it will make the movies fit less seamlessly together.
The MCU often tries to connect its stories by having characters appear in multiple movies other than just their own. The crossover is part of what makes the MCU fun, and it's the main draw of an Avengers story - watching a team of heroes with different personalities and powers come together. And when it's done well and not overused, it really works. But it often feels like MCU characters make cameo appearances in other movies just for the sake of it rather than actually fulfilling a purpose within the story. Additionally, character introductions feel rushed now, as major characters from the comics will appear with little to no introduction as a cameo or in a quick after-credits scene as a way to tease another upcoming movie.
What it boils down to is that every Marvel movie now has to set up every other Marvel movie that's coming out after it. None of the stories are allowed to exist on their own and they all have to reference each other as much as possible. This is a thing that happens in other franchises too, of course, like the DCEU, but the MCU is the biggest and most egregious example of the perils of a widely interconnected universe. There are a lot of benefits, and it can be really rewarding to watch these stories pay off and see characters interact, but it's starting to come at the expense of any sense of individuality and uniqueness within the MCU. When every movie has to fit the universe, they all start to feel like one long movie that never ends.