For creating a quick and dirty project it's great. But after a while you find you end up using less and less of bootstrap because moulding it to your requirements becomes a burden. That's prob why you rarely see big complex sites using it.

There's nothing worse than using react or angular and then someone pulls in a library to have a bootstrap control that breaks stuff and doesn't work and is hard to style. Spend hours trying to get it working properly beyond the most basic implementation of the control.

I prefer bulma because it focuses more on the basics and doesn't get in the way. Even after you create stuff you can shape and mould t easily.