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.