Bootstrap is cool.
I've been using Bootstrap for about 5 years. In my current role, I've set up a tier system:
The reason I set it up this way is so that I could possibly upgrade to newer versions. Since Tier 1 is 99% untouched, I can pull in a new version and change that 1% - if that's how the newer versions work.
Depending on the use case, the problem with Bootstrap is that it gives you everything whether you use it or not. This can bloat applications. It also packages certain things up (jQuery, Popper, whatever other libraries), making it difficult to extract just what you need.
A quick-and-dirty alleviation to this is to use the CDN. Obviously CDNs are fast, but it's not unlikely that the user may already have the file cached, since Bootstrap is extremely popular.