By my count, there are at least four distinct movies in Black Widow, two of which I really enjoyed. If you're thinking that's a roundabout way of saying the film as a whole is something of a mess... well, you're not wrong. But I mean what I say about the good stuff: when this works, it really works. Of course I can't really talk about the component parts without at least acknowledging what they are, so consider this an extremely longwinded and needlessly confusing spoiler warning.
My guess is the script for Black Window went through quite a few iterations before reaching this point. It feels like a movie that started with a strong vision that got chipped away by studio notes and rewrites. The first act goes in some dark directions: we get some backstory and insight into Natasha and the world she comes from. Tonally, this clashes a bit with the jokes tossed in to maintain the MCU flavor, but it still worked for me. I thought it did a great job selling this as a darker corner of the same world.
After a few action sequences (more on these in a minute), the movie transitioned into a sort of Soviet superhero sitcom for a while. I really liked the spy thriller opening, but this... this I absolutely LOVED. It was weird, funny, and surprisingly touching. The characters were emotionally broken people, and it was a joy seeing them interact in almost a parody of 80's situation comedies.
You've probably already done the math, so here comes the stuff that disappointed me. First, the action. It's not so much that it leans heavily on CG and blue screens, or that it doesn't look real: that's true of a lot of the genre. The larger issue is it doesn't feel integrated into the story. The fights feel like distractions, and not particularly interesting ones. It doesn't help that most of the antagonists aren't distinct or interesting enough to be compelling. The only one with a personality stays offscreen most of the movie, and the rest are literally mindless puppets. This becomes a pretty big issue towards the end, when an army of nondescript enemies works against the theme they're supposed to represent.
And speaking of theme...
Here comes that "fourth movie", and I need to be very, very careful, because I have a feeling I'm going to be in bad company here. Black Widow tries to work in some social commentary, and it just didn't work for me. Before I go on, I'll acknowledge I'm a cisgender man, and it's entirely possible this will play better for other audiences.
To be clear, I think what Black Widow tries to say is good, and I like the concept behind how they're trying to say it. But I don't think the idea was given enough room to breathe, particularly because it was competing for screen time against themes of family, and those felt more developed. Themes built around misogyny and control came across as rushed and didn't deliver enough of a punch.
Ultimately, I think this was one of the MCU's weaker entries, though that's a long way from a failure. We still got great characters, including several new additions I'd like to see again. It's enjoyable enough to warrant a viewing or two, but aside from the aforementioned sitcom sequences, it's not all that memorable. It also raises the question why Disney+ is charging an extra $30 bucks for this but giving us far superior Marvel series free with subscription.