Not having to support the OpenSSH / OpenSC / GPG stack is a big one for me. The built-in support shipped on MacOS just works[1], Putty-CAC just works on Windows, but the OpenSC + GPG setup is semi-stable and will randomly fail in ways which require you to stop what you're working on to kill daemons or unplug/replug the device to get it working again. I would not want to support anything involving that stack.
1. The Yubikey docs describe the pre-Sierra process — https://support.apple.com/en-us/HT208372 has what most people will want to use after 2016. See https://github.com/Yubico/developers.yubico.com/issues/197.