I don't think anyone's established a definitive list. Most MKVs contain an x264 video stream and AAC audio, but the precise nature of the data contained in these streams varies. I have seen suggestions that the Hummy rejects some VBR (variable bit rate) streams and 5.1 audio, but others may know better.
Recently video files obtainable from certain web sources have adopted new standards which the Hummy cannot handle. I am fortunate in having a media PC permanently connected and I now find it easiest to play all media not originating on the Hummy directly from the PC, particularly since I discovered that attempting to play some files does not merely fail, but actually causes the Hummy to spontaneously reboot. I have tried recoding some files the Hummy rejects, just to see what works. The results just add to my puzzlement: for example recoding a MKV file to AVI with x264 video at 720p and mp3 audio produced a file that played perfectly on the PC but stuttered badly on the Hummy.
It's also worth mentioning that most DLNA servers appear to ignore MKV files completely, so streaming is a non-starter.
I've been discussing this on another forum and someone came up with a method that seems to work well, though I can't vouch for surround sound as I don't use it.
Open the .mkv file in tsMuxeR (download here). Select Output to be TSMuxing and hit the Start Muxing button. The result is a .ts file that plays on the Humax or on the PC with Splash Player or VLC. Time to mux a 1.4GB .mkv file: approx. 30 seconds.
This works for me with the first couple of problem files I've tried - ones that caused the Hummy to reboot. Please feedback here if this works/does not work for you. Also, I have not tested it, but I'm wondering if this will allow HD files to be streamed with the Humax media server.
Does seem weird that it works one way, but not the other.
It's not a major issue, I can connect a USB hard disk to it.. but it would be better if I could do it from my NAS.
Wonder if the NAS 'presents' the file differently. I may set up a media server on my PC and try it from that !