Semantic UI is ~0.5MB minified while Skeleton is ~20KB.

Aside from what has already been said (SemUI is a MASSIVE piece of CSS), Semantic UI is also not semantic.

<article> is semantic <div class="article full width"> is not semantic, it's describing how it should appear.

You can break half of the examples page by just clicking everywhere and typing and such. I currently have three buttons stuck in the middle of their two positions, textboxes being broken, etc. It needs work.

Also it's rarely a good thing if your goddamn landing page makes an i3 stutter when rendering it. Doesn't really scream lightweight or efficient (although the biggest culprit is that background header video)

However, Semantic.gs is pretty good. Minimal and much more semantic.