(also many top N companies do use 3rd party code, like React, but they usually re-host that themselves)
I still get messages along the lines of, "Your hard drive is full. Dropbox cannot synchronize until you free up some disk space," with 4 GB free, even though many users have complained loudly about that for years. The Dropbox client was also causing weird problems with dropdown menus in MS Office applications for a long time, which took forever to track down. That at least appears to have been fixed now, although I can't (or perhaps don't want to) imagine how it happened in the first place.
Dropbox does hard things, HN conventional wisdom notwithstanding, and they do them reasonably well. But they can be very slow to recognize when they're doing something wrong.