TLDR in bullet-point format (I know this post is getting long, but its shorter than watching forty minutes of video [Even though, its a super fun video to watch])
Steven Universe is a good show. But it's not a GREAT show. This is largely due to their use of filmmaking techniques only on their most basic levels, avoiding much variety or advanced techniques that would keep the show visually interesting.
Characters frequently seem to forget their own abilities, mostly for the sake of plot convenience. Personalities and experiences also vary, with one example being that Amethyst states she has been to a ton of parties, but later has no clue how parties work.
Art style consistency is a mess, with characters frequently off-model (color changes, height differences, clothing, etc)
The show doesn't recognize whether it's an action or a comedy. Many characters, while comedic, also are very unsettling, with some characters being emotionally manipulative or near homicidal at times. As such, characters feel inconsistent in their personalities. Are we supposed to take them seriously? Or are they comedic-jokesters?
The show being in Steven's perspective only tends to make the show monotonous, because if the writers want to focus on another character, Steven has to be forced in, no matter how convoluted that ends up being. This draws attention away from the characters they're trying to develop in the first place.
Steven Universe is on Season 4 and almost nothing has happened. Almost every arch resolves with the world going back to normal, rarely does an event occur that changes the world around them, and new characters do almost nothing than just...exist. They don't push the narrative. Almost everything so far could have been accomplished in Season 1. Characters also seemed to be ignoring a giant monster that lives in the crust of the earth that could destroy the world at any moment, because that would ruin the atmosphere of fun. Then, when that resolves, its back to business as usual. Nothing changes.
No main antagonist. The Diamonds rarely ever show up, and don't do anything when they do. Antagonists need to be introduced early on, and yet they didn't show up until Season 4. They had only been mentioned before that, which is alright, but they had no presence at all until Season 4.
And finally, the uploader is going to bail on the show if they make that whole "Rose didn't actually kill Pink Diamond" thing canon, because, come on.
(Whew. Tough job, but it's done now.)