Bootstrap is developed to be instantly compatible with all sizes of screens, so you don't have to worry about which device the user is accessing your site from. Yet if you prefer, you can disable responsiveness of Bootstrap.
The grid layout with the predefined CSS elements and JavaScript components make it easier to have consistency across different browser versions and even different devices.
Documentation is thorough, well organized and full of live examples and templates ready for use. Every component and every part of the framework is explained and covered in depth.
Bootstrap is designed to get a site up and running quickly. Each of it's components is pre-configured to help with getting a site up and running quickly.
Bootstrap is very popular and has a large community. As a result of this it is much easier to find help with anything you might need. This also gives you a treasure trove of prebuilt components to use and add to your site.
Bootstrap's over-reliance on HTML classes for styling can get very messy very quickly. There's also an overabundance of DOM elements which have a lot of classes and are more often than not nested inside DOM elements with even more classes. This gets problematic down the line because the maintainability of the project gets harder when the project starts to get large.
Bootstrap is very popular and has a large community. As a result of this it is much easier to find help with anything you might need. This also gives you a treasure trove of prebuilt components to use and add to your site.