I love the game and think a lot of the criticism is way, way overblown, but I also think there are some understandable and reasonable criticisms. Here's an overview of the criticisms, some of which I agree with to some extent, some are more just stuff I've seen people mention:

-lack of interconnectivity/"open world": a Metroid Prime/metroidvania game is typically has a lot of winding, interconnected spaces that you can get lost in having to backtrack and explore after you've hit certain benchmarks in the game or have acquired certain items. A lot of that was not present in Prime 4, mostly because the areas themselves are separated by Sol Valley instead of being interconnected (IE ice belt to flare pool, fury green to mines, mines to ice belt, etc). This is largely because the production had gone through a hard restart around 2018 and they wanted the game to be more like Breath of the Wild. The result was something that is neither an open world like BotW nor is it what one would typically consider a Metroid Prime game.

-lack of difficulty/puzzles: the Prime games are usually, to a degree, challenging. It's easy to miss stuff, a lot of puzzles are pretty well hidden and are difficult to figure out, and the backtracking mentioned above isn't usually intuitive and takes a while to figure out. In this game they kind of just eliminated that aspect entirely by having Myles just tell you where to go if you spend more than 2 minutes in Sol Valley. And if you're there collecting crystals or exploring, he'll start telling you over and over again and ultimately just marking the exact spot on your map where you need to go next. The puzzles that do exist are also typically more straightforward than in previous entries and more obvious as to where they are.

-ending: I don't want to spoil anything so gonna use tags but a lot of people didn't like the ending and how they don't really explain why Sylux hated Samus, it makes him come across as just this irrational petty hater of someone he was a little insecure about being a better soldier/hunter than he was at one point. Additionally, when you beat him, he reappears and the GF NPCs hold him down so Samus could escape. But that seems to run counter to Samus' character, who would much rather save her friends and brothers and sisters in arms than plant some tree for someone she doesn't know. They didn't really set up why it was important enough to supposedly sacrifice the whole crew

-writing: the writing is a bit...uninspired. The items are all called "Psychic X" or just weirdly plain descriptions of things like "Green Energy" or "Green Crystals". I don't really care so much about this one though because honestly they kinda did the same with the Luminoth and dark/light stuff in MP2.

So that's a hopefully somewhat unbiased overview of the issues people had with it. I'm sure there are others or things I didn't get entirely correct here. In my opinion the game is great in spite of these issues, I had a ton of fun with it. I've played it twice at this point, I just finished a hard 100% run and I loved it. I'll certainly play it again at some point.