I haven't really seen Picnic CSS in the wild (only here https://gyshido.com/ and in my own projects) so I actually have no idea on adoption, which is one of the main decision points. I will probably keep using Picnic for most of my projects.

My two main alternatives are Materialize CSS for mobile because of the styles ( http://materializecss.com/ ) and Semantic UI for quick mock-ups/prototypes because of how complete it is (while looking way better than Bootstrap). Another "alternative" (not for CSS but for the type of projects I do) is https://html5up.net/ since they are really well designed when personalization is not so important.