Unless you're using bootstrap components as-in-the-examples in the same context and elements, they work fine. But try using them somewhere, like an input or button in a table cell, or card in a grid, etc - and you'll spend an eternity of "unsetting".

Basically bootstrap is not composeable. It provides not just a set of styles - but also a severely restricted visual design. People quickly run into elements that bootstrap was just not designed to be placed into. That's why there are so many forks/extensions of bootstrap, like bootstrap-admin and so on.

So its good to start with and follow the official style guide. But doing anything more, and the restrictions immediately bubble up. And then you quickly run into an entirely different set of problems on organizing your own CSS, like using SASS, BEM, etc.

Its not a question of OOCSS vs BEM, but more of how tolerant/accomodative the framework is in using it in different elements and contexts. Bootstrap's tolerance is relatively low compared to most other frameworks.