Working with bootstrap on a few projects I noticed it is great when you use it’s components as they are but not so much when you have to modify them beyond editing the variables file which lets you change mostly colors. If you want to do more fundamental changes you’ll need to rewrite the component in part or entirely. And, that’s exactly what will happen if you are a perfectionist or want to make a highly responsive web app or simply a design that doesn’t look like bootstrap.
A front-end framework is a bunch of interrelated components meant to facilitate the creation of websites by assembling those components following speciffic rules.
Micro: Buttons big small normal | red green | icon (left & right) | link | block classes should be combinable.
Forms: inputs, select, checkboxes, radio,
icons : Svg file
Structural layout : a way to set the scaffoldings and all else
Typographic type : Set the type for headings and for body text as well as small text
Everything else that comes after is mainly custom parts. Nothing more nothing less.