> I have seen no evidence of this in practice. This is only true if you design is basically a skin of these frameworks. If you have anything remotely custom you end up overriding so much you might have well just started with a normalise/reset style-sheet. So for some internal application that never seen on the outside, you can use bootstrap and slap something together quick sure. However if you are actually building something with a unique it just doesn't really work.
I had the opposite experience. I've build a few projects with Tachyons and Tailwind, and I rarely have to create custom CSS. In one of the largest ones (that actually had a 100% custom UI toolkit with over 40 widgets) we only had about 120 lines of custom CSS, pretty much just overriding colors and adding some utility classes for animations, custom drop shadows and some weird hover states.
The reason we need so little extra CSS that is that those frameworks use the same defaults that designers use: everything is a multiplier of 4px/8px, font sizes are proportional, and it follows design principles. Actually, if you work with a designer chances are most of your CSS is already very repetitive, because designers love patterns.