Having used both, I find that although Semantic UI has a lot more pluggable components, documentation on anything beyond simple examples and one sentence descriptions is severely lacking. Some components are just plain buggy, and the developer is not very fast at fixing them. It's got a lot of features but the lack of polish and development speed makes it hard to recommend for a larger team.
I think only having a single developer has had mixed effects on the project, which is a shame because it's so close to being great.
Meanwhile BS4 has solid built in components that are just documented enough to be usable despite not having as many. Issues get fixed quickly and it feels easier to follow its conventions. The JS APIs are simple enough as well, and the code behind it is much easier to follow than in Semantic UI.
Yes Semantic UI has come a long way and changed since I first used them 1-2 years ago. I have just tried it on a new project their annoying build process and lack of official SASS support the style and out of box components are really great.
Semantic UI looks the best, but once you start to do your own things, you end up fighting it. I'd love to see some of the stuff Semantic UI does just as an add-in/theme for Bootstrap (Forms are beautifull).
I created an app for a customer in it, and while it looks shiny, it would have been more cost-effective to just have used bootstrap.
I've been helping out a developer that has been building sites with Semantic UI and it's making my skin crawl.
It seems like a good start for prototyping and admin interfaces, but for anything else it just adds too much styling and classes that need to be overridden.
The name seems poorly choosen, since you're constantly adding classes that reference styling decisions making it everything but semantic (ui very wide, ui very thin, ui three column doubling grid, …).
I think only having a single developer has had mixed effects on the project, which is a shame because it's so close to being great.
Meanwhile BS4 has solid built in components that are just documented enough to be usable despite not having as many. Issues get fixed quickly and it feels easier to follow its conventions. The JS APIs are simple enough as well, and the code behind it is much easier to follow than in Semantic UI.