I get the argument that fully custom scroll bars aren’t great for accessibility in many cases. However, the default behavior out of the browser or OS is pretty terrible in many cases as well.

Scroll bars popping in and out of existence awkwardly reflowing elements around it. Tiny unusable 30px wide horizontal scroll bars inside an input element because an element overflows 0.7px due to font size rounding issues or what not.

Default scroll bars are great for documents, but pretty terrible mostly for slightly more complicated UIs where a lot of info is displayed in small area.