For the h264 licencing cost, read this ~10 year old article and you will understand
No idea about the warning on mint though. It may mean the playback on mint's default media player, which is probably some gstreamer based app, so it may just install gstreamer1.0-plugins-bad and gstreamer1.0-plugins-ugly. I do not use the gstreamer backend on any app, so I have no idea what each package contains and why they are called like that.
Anyhow, if it is gstreamer related, it won't affect opera and chromium.