CSS framework for desktop and mobile apps

Just wondering what experience everyone has had with the various CSS frameworks that support a mobile first approach such as Bootstrap etc.

I’m going to be starting a project that needs to support web and mobile apps with Meteor and would like to start out on the right foot with a good CSS framework. I recently read a post where someone switched from Bootstrap to Polymer mid project because of issue but didn’t specify which issues they had.

Are there any gotchas as far as size, complexity or compatibility between devices that I need to know about up front?

Anything else to be wary of?

Any advice is greatly appreciated.