I recently pirated the Superman 2025 movie as a .mkv which loads captions perfectly fine on GNOME Video Player but my friend couldn't get captions working using VLC on Mac.

Which easy to use media players can I recommend to my friends on Windows and Mac to ensure mkv captions work out of the box for them?

MPV is pretty much a gold standard for video players, it's a bit more keyboard focused, but basic functionality is fully accessible through the GUI.

But I don't think VLC should struggle with embedded subtitles. Maybe you can feed your video file to ffprobe and share the output for a bit of troubleshooting?

VLC isn't lacking support for this. Are you sure your friend knows how to turn on subtitles?

I had some issues with mkv files on VLC a few weeks back (same files worked fine in Haruna), but they seem to be working ok for me now.

Weird. VLC subtitles on mkv files work for me under Linux. I'd recommend troubleshooting VLC instead of searching for a different program