<rant>
They're overused, bloated, and in my experience, have created more problems/headaches than just writing my own code. Your experience may be different, and that's cool, but I've spent more time troubleshooting why something is happening than anything else when I've used a framework. I think we've reached a point where people are getting too far away from code. I'm guilty of it myself. I've been using jQuery so much, I'm pretty sure I'd need a refresher course to write regular JS.
Frameworks also add a good bit of bloat to your code. Most coders don't use half of what's in the framework, which means users are downloading hundreds of kb in code that doesn't get used. "Only ##kb minified and g-zipped!" the sites say. How many front-end coders really use or even know how to use server-side g-zip? I know how, and I've used it exactly once, just to try it out.
</rant>
All of that said, I've never used Bootstrap itself, so I can't speak to that framework specifically. However, I've used a number of others over the years, and they're why I won't bother with them anymore.