For the end user, its main weakness is that complex pages can be pretty slow to render if not coded well. It's not that bad either. You wouldn't be like "oh this is a React site, yuck", they're all like that these days for the reasons you'd expect.

As for React Native, its main issue is the communication between the JavaScript browser-ish environment and the Java/Kotlin native environment that can be costly because every has to be serialized (meaning, converted to some type of data structure both sides can understand) and deserialized, so complex screen updates don't scale too well.

It's easy for developers to accidentally trigger much bigger and much more expensive rerenders than expected. If you see whole second long page hangs on some websites as new content loads in that's usually what happened.

For developers, it's complicated, you kind of need to experience it to understand the footguns.

React was born to solve one particular problem at Facebook: how can we make it so any developer can jump on any part of the UI code and add features without breaking everything. One of the most complicated aspects of a website is state management, in other words, making sure every part of the page are updated when something changes. For example, if you read a message in your inbox, the unread count needs to update a couple places on the page. That's hard because you need to make sure everything that can change that count is in agreement with everything that displays that count.

React solves that problem by hiding it away from you. Its model is simple: given a set of inputs, you have a function that outputs how to display that. Every time the value changes, React re-renders every component that used that value, compares it with the previous result, and then modifies the page with the updated data. That's why it's called React, it reacts to changes and actions.

The downside of that is if you're not very careful, you can place something in a non-ideal spot that can cascade into re-rendering the entire page every time that thing updates. At scale, it usually works out relatively okay, and it's not like rendering the whole page is that expensive. There's an upper cap on how bad it can be, it won't let you do re-render loops, but it can be slow.

I regularly see startups with 25MB of JavaScript caused by React abuse and favoring new features over tracking down excessive renders. Loads the same data 5 times because "this should only render once" and that turned out to be false, but it displays correctly. I commonly see entire forms being re-rendered every character you type because the data is stored in the form's state, so it has to re-render that entire tree.

But it's not that bad. It's entirely possible to make great and snappy sites with React. Arguably its problem isn't React itself but how much it is associated with horrible websites because of how tolerant to bad code it is. It's maybe a little bit too easy to learn, it gives bad developers an undeserved sense of confidence.

E: And we have better solutions to this such as signals which SolidJS, Vue and Svelte make heavy use of. Most of the advantages with less problems.

Anyway, that part wasn't relevant at all why I don't like React. The point is, skip the web, you don't really need the web. React Native skipped the whole HTML part, it's still JSX but for native app styled components for UI building. The web backend worked very well, your boxes became divs with some styles. It pretty much just worked. Do that but entirely in Rust since Rust can run natively on all platforms. Rust gets to skip all the compromises RN needed, and skip the embedded browser entirely. Make it desktop first then make the web version, it'll run just as well and might even generate better code than if a human wrote it. Making the web look native sucks but making native fit web is a lot easier than it looks. Letting go of HTML and CSS was a good call from React Native.