Although UI Frameworks is as hard to learn as a new programming language, there are many great reasons to use them. Using a UI Framework like Semantic UI allows the user to work fast and efficient. Although it gives a headache when learning to use them, Frameworks saves time in the long run. In the case of Semantics UI, there are many lines of code that is easily added, such as a gridline or buttons. It is also very easy to debug compared to coding normally. Using a theme is easier because Semantics UI makes the theme easier to inherit by the rest of the code.
It is better to use UI Frameworks than HTML of CSS because the frameworks because it takes care of all the little parts of the code that is tedious to code with just HTML and CSS coding. It keeps the code organized so others can understand the code easier. If a developer decides to leave a project and a new developer is picked up, then the new developer will be able to understand the framework better than HTML or CSS code that was made up from someone. Frameworks are used by people all over the world, so there will most likely be a fix for your problem somewhere online.
Based on speculation from what other people have told me or what I read online, it isn’t the hardest to use. One person said that it was less restricting than Bootstrap. Although it can be somewhat hard to learn and use, it is worth it in the end.