The problem with CSS has historically been browser conformance—naturally there’s been some workarounds. However, CSS has been a success because of its ubiquity. However, it is somewhat limited in its extensibility.
For app development I really appreciate Qt Quick QML as it’s really nice language for UI development, but also highly extensible. Granted, different use case than CSS in ways.
Bootstrap was effectively the jQuery for CSS, in terms of standardizing what layouts looked like across all browsers.
It gets a bad rap because it's so easy to use for non-designers that a lot of sites that use it don't bother customizing the UI elements like buttons or expand/collapse accordions etc. Or the layout of the page, for that matter. The famous "Every Bootstrap website ever"[0] site outlines this nicely:
