The idea is that sometimes people overuse them. Like, putting borders EVERYWHERE.
Even HN does't have border for the background.
So of course sometimes they are okay. Bad thing is that when you have too many lines. E.g. if you have a bordered table, bordered table filters, bordered buttons, bordered icon buttons etc..
I do agree with you, but I have a real issue with the current trend of UIs where everything is just text & icons thrown on the same panel (like here in HN, or the latest macOS). I tend to miss the old paradigms where applications had/tried to respect a minimum of visual consistency with others and things such as OS-wide theming allowed to adjust ALL applications to your taste and visual abilities.
Even HN does't have border for the background.
So of course sometimes they are okay. Bad thing is that when you have too many lines. E.g. if you have a bordered table, bordered table filters, bordered buttons, bordered icon buttons etc..