CSS frameworks are very powerful tools, they can speed up the process of building your site but it can come at the cost of your site looking like many other sites out there if you don't take the time to go fully in depth on customisation.
Take bootstrap for example, when it was introduced everyone went mad for it, I remember it was something potential clients would always ask about, but back then it resulted in so many sites that looked the same, it's different now and more customisable so this doesn't happen as much.
What has also happened is Frameworks have grown so much and because more feature rich that they are quite bloated with features you might never use.
At my current agency we use Foundation, it's not as feature rich as Bootstrap but that's ok because we mainly use it for it's grid layout and some of its SASS functions (rem-calc is a dream), it's a small file size and because we use it with SASS it's super easy to strip out what we don't need.
If however I'm working on something small or a personal project I'll normally just write everything myself, I often find that I don't need much outside of the base element styles and a simple flex or grid layout. This can help keep the size of your CSS down and improve your performance.
It's all about deciding how much you need from how much you want to build yourself, are you looking to save time or ensure everything put into your styles is there for a reason to keep bloat down.
