Pre Bootstrap / Material / whatever - I'd end up with designs that were just incoherent. You'd look at a modal, or a file upload or something and none of them individually were that awful, but as a whole they didn't fit together.
When I use things like Bootstrap it's as a scaffolding in the early stages of development before I write enough of my own CSS. Eventually then I've restyled all the Bootstrap components and I only keep it included to avoid spending 10 minutes re-implementing modals in JS.
If it was user facing, I'd typically use a theme built on Bootstrap.
From this perspective, Bootstrap is basically an API you can develop against.
For customer-facing products and websites (where you can have actual designers involved) it's maybe not as compelling, or even a negative ("oh look, a bootstrap site"), but it's fundamentally not a worse choice than any other framework or programming language.
Then CSS Grid finally became available accross all major browsers. I'd never used it, but decided to try it and managed to have my layout working the way I had always wanted it to in a couple of minutes. In fact, it took more time for me to explain to my concerned wife why I suddenly burst out howling and laughing like a lunatic than it did to actually swap out flexbox for CSS-grid.
Still using bootstrap for a good portion of the UI, of course :)
If grid has a way to address these unpredictable divs (not knowing how many there will be) then my last use case is closed.
For my use case, I'm using CSS Grid on this project (https://www.contabulo.com) to manage the layout of 'cards' on a 'board', where the number of cards is variable. I was originally using flexbox, which proved to not be.. flexible enough to manage it properly (and there were some annoying behavioral differences between browsers).
I was worried I was going to have to resort to some Javascript-hackery when I happened upon CSS-grid being mentioned in an HN post a while back. Luckily, support for grid was added to all of the major browsers not long after I discovered it :)
Actually, this has come to be the part of bootstrap I like the least. Individually the components are extremely useful, but as a layout engine I've constantly struggled to get it to comply with anything but layouts that look a lot like the ones in the bootstrap examples. CSS Grid and flexbox have proven to be more flexible at composing bootstrap components.
But I'm curious to your statement about "working together". Are Bootstrap components "aware" of the layout css/components?