Semantic UI was difficult to use at times. Sure, it implemented many features that I really liked. I’m thankful that I don’t need to find a million different icons for every single UI element with an icon.
With a UI framework, most of the coding behind design features such as the beveling of edges are done for you. This means that you can avoid most of the coding and instead focus on the actual design of the site. Coding raw HTML and CSS requires much more work on the finer details.