Bootstrap doesn't care about your environment, can be abstracted easily into automated Grunt->Assemble generation (amongst other flows) using JSON data as the i

Bootstrap doesn't care about your environment, can be abstracted easily into automated Grunt->Assemble generation (amongst other flows) using JSON data as the informer, works with an immense amount of different use cases, has highly active forks with SASS rather than LESS (if that's your flavor), and doesn't care about your development model regarding JavaScript. In effect, it's neutrally-opinionated on a high level. Gumby might have its merits, but if I'm working with a set of fairly green developers new to the industry, I'm not throwing a highly-opinionated system at them to back them into a corner.