I'm not sure. The bar for UI has been raised significantly since Facebook launched and mobile has changed the game to such an extent that you need a presence on 3 platforms out of the door (iOS, Android & web) to even stand a chance.
Not entirely true for many apps. The app I work on does not need a working mobile site, because we don't think our consumers are going to use it on mobile.
However, we do very much care about our UI - and I think react at this point helps with that. But I don't think React complicates the stack too much. It may even make things easier, tbh - I don't have to test pages on backend tech, just endpoints.
React easier compared with what - imperative PHP4 at Facebook in 2004? I'd have to disagree. React state management is much heavier than returning HTML from an embedded PHP template.