Recs.
Updated
Specs
Pros
Pro The elements offer a huge amount of customization, far beyond a framework like bootstrap
Pro Beautifully designed
Semantic UI has a futuristic and beautiful design. Many will satisfied with the design, especially when Semantic UI is used as a CSS framework.
Pro Easy to use
Just start code from the beginning of the journey, from first page of documentation.
Cons
Con Not for beginner developer/unfamiliar with Javascript
Many features in Semantic UI uses Javascript customization such as for Modal. This is unlike Bootstrap that can add Modal just with customizing the HTML attributes. Developers who plan to using Semantic UI must be familiar with Javascript or JQuery to get the most out of it.
Recommendations
Comments
Out of Date Pros + Cons
Con Subpar JavaScript plugins
While the styling and components parts of Semantic UI are well-made and eye-catching, the logic and JavaScript part needs some work.
Many features are bugged or behave in unexpected ways. Some aren't optimized to work with all devices. For example, the navigational menu in the Semantic UI website is often laggy when viewed through a mobile device.