The details are pretty well covered here
https://github.com/Homebrew/homebrew/issues/23333
TL;DR is that with DisplayLink drivers installed (even as of 2.4b1) it installs a file at /usr/local/lib/libusb-1.0.0.dylib
This file conflicts with Homebrew's 'brew doctor' command and results in a spurious error
Not the end of the world but maybe there's another place DL can place this library so it doesn't conflict with Homebrew?