Why are you assuming that the causal factor is with the framework?
The reason they're shitty is because they're heavier weight and "mobile first" usually comes with a certain amount of tradeoffs for other devices. React in and of itself as a means of managing data flow is very elegant IMO.
It just happens that if you tell any random group of engineers to do a full rewrite, they're going to settle on whatever is currently popular.
One factor that the framework makes worse is linking and tabs. The further everything goes towards single page apps, the harder it is to bookmark things or open them in new tabs. The frameworks people use to make single page apps usually come with tools to give everything urls and solve this problem, but using those is more work for the devs, so the tendency is for things to become harder to open in a new tab or come back to.
Either the question is disingenuous, or you just don't understand that the choice of UI framework (such as React) has no significant impact on the visual design of a site or app. There are caveats, because certain paradigms can make implementing certain designs more challenging, but for the most part they are unrelated.
Now you are assuming he doesn’t know what he’s talking about. The choice of framework can certainly affect performance, especially for UI interactions (which is the case with reddit), and might encourage patterns (for example, async loading everything) that can have significant impact on the user experience.
Instagram, Twitter's mobile site (mobile.twitter.com/home), React's own documentation page, Facebook itself, Wolfram Alpha, CoinBase, WordPress, CodeAcademy, Atlassian, http://builtwithreact.io/ thousands of other sites. What a weird and lazy comment. UX is completely orthogonal to the tech driving it except where performance is concerned.
The reason they're shitty is because they're heavier weight and "mobile first" usually comes with a certain amount of tradeoffs for other devices. React in and of itself as a means of managing data flow is very elegant IMO.
It just happens that if you tell any random group of engineers to do a full rewrite, they're going to settle on whatever is currently popular.