What existing CSS frameworks have you used locally, or in production?

Bootstrap: Slow release cycle. Bootstrap 4 was in alpha for almost two years. Future versions of Bootstrap should include a spinner button component, as it is widely used.
Semantic UI: Source code structure makes theme customization extremely hard to understand. Its unconventional theming system is a pain to customize. Hardcoded config paths within the vendor library. Not well-designed for overriding variables, unlike Bootstrap.
Bulma: A lot of non-semantic and superfluous classes and markup required. Not backward-compatible — upgrading versions breaks the app in subtle ways.