Unfortunately .mkv is not well supported by some macOS apps.
In order to share the file, we often need a conversion from .mkv to .mp4. A simple container change using ffmpeg does not seem to work correctly (file does not open in QuickTime, lags in other software, etc.) so we actually have to re-encode the file, which is an error-prone and time consuming process.
For the sake of interoperability, it would be very nice to support the .mp4 container as output of the blender_video_chunks job type. The major drawback of the .mp4 is the loss of any metadata that .mkv could provide (once we will start taking advantage of that) but I would consider that acceptable.
Thanks for your consideration!