I honestly just find the extra layers to make it harder for me to know what my code is doing. I'd rather set proper CSS margin-bottom than mb-1. Having to learn the Bootstrap way of doing things when I already know the traditional way mostly feels like a waste of my time. It's not, but it's hard to stay engaged when I can already do a thing in a more standardised way.

Kind of like a site I've been stripping the jQuery out of. You don't need that to show/hide a couple of form fields, FFS. Or the special JS library for doing pop overs. Come on, there's three fields on the entire website that use them, just use HTML5 popovers.

I imagine Bootstrap is probably more useful for stuff where more complex layouts are needed, or when a site needs to be more responsive to different browser shapes (as in desktop vs. various mobile form factors).