- Standard layouts
- Standard input and display components
- Standard event and message handling
- Standard user interactions
- Standard color themes
Which is great for developers to set up an initial UI, but makes every application look the same and takes a lot of work to customize in a way that's consistent with the defaults.
On the other hand, building a complex, feature-rich UI requires a lot of skill both in terms of implementation and design, which is less than common. So that Qt UI will be functional and predictable to users, even if it was not very well thought out, and the broken web UI will be useless to everyone.
Even after reading lots of UI guidelines, and developing several tools and applications, it's taught me that I'm terrible at it, and should let an expert do that part while I stick to the other parts!
Not actually UNIX UI libraries.
(take my experience with a grain of salt, I've only used React on side-projects, never anything complicated or that I was forced to develop on because of work, and never ran into any perf issues)