I would say neither, because how they work and what they do has the nasty habit of destroying any chance at a properly accessible semantic deployment. they are a stunning example of taking something simple -- building a website or application UI -- and making it as convoluted, painful, and useless as possible.
They often use client-side scripting to do things HTML and CSS don't need scripting help to handle, concepts like CSR (client side rendering) in a manner that breaks accessibility and depending on the business type could land you in court for US ADA / UK EQA violations, and typically result in using two to ten times the code needed to do the job.
Everything I've ever seen done with web technology front-end frameworks falls into at least one of the following categories.
1) Things that break accessibility.
2) Things that could have been done faster, cleaner, simpler, and in a more sustainable fashion in a fraction the code, not even counting the size of the framework against the total.
3) Things that have zero blasted business on a website in the first place.
Which is why generally I am of the opinion that those who choose any sort of front-end framework -- be it HTML/CSS like bootstrap or w3.css, JavaScript like jQuery, or hybrids that also screw around server-side like react/vue -- simply haven't learned enough about HTML, CSS, or JavaScript to have any business building websites/web applications for business. You want to use it for a blog for grandma, fine. But I go into one more government agency, bank, public utility, or healthcare company facing daily accessibility fines because someone slopped things together with one of these {string of expletives omitted} frameworks, I'm gonna puke.
My current freelance work consisting almost entirely of ripping this trash out of business websites that have been raked over the coals by these {another string of expletives omitted} frameworks!
They are at best an ignorant choice made by those who haven't learned any better. Again, not meant as an insult; ignorant means you don't know. We can fix ignorant.
At worst though? Predatory scam that propagates and sustains itself with two-faced lies, and propaganda techniques like bandwagon and glittering generalities. The people actively promoting and deploying these systems for "business" looking at the average non tech-minded executive like a hyena who sees a baby lamb all alone in a field.
Do yourself and your business a favor, forget any of that nonsense even exists. Just use the underlying languages the way they were meant to be used. Every single claim you hear about such frameworks making things simpler, or easier, or "better when working with a team" is a bald faced lie! A lie painfully apparent to anyone who's taken the time to learn what semantic markup means and how to use it, how to maintain separation of presentation from content, and the most basic aspects of accessibility.