It's best to think of .MKVs as not working at all, frankly. When you're downloading files in .MKV format you usually have no way of knowing their bitrate. Its pretty clear that nobody here who is having trouble is ripping their own .MKV files. And 9 times out of 10 if an .MKV will even start on an Xbox, it will play unacceptably badly. The Xbox technically supports .MKV, but in practice it doesn't.
Oh no, the Xbox1 running XBMC fully supports the MKV and will run any MKV you throw at it... Unless the CPU demands are too high. The vast, vast majority of encodes use MKV so they can use the MPEG-4 AVC/h.264 video codec and the AAC audio codec. These are about as complicated as video can get and AVC/h.264 has various 'profile' of which features are enabled which determin how much CPU power is used. Those encoders, targeting PCs for decoding your video, set it to High Profile, if not higher, to maximize encoding efficency at an expense of CPU time needed to decode it. The MKV itself is just fine, it's the fact that most 480p h.264 content, especially scenes with a lot of motion will burry the needle on the CPU. Try to run 720p stuff? That's just flat out not happening.
There's nothing with with MKV and if you load DivX/XviD into an MKV container and it'll run perfectly on the Xbox1. h.264? No, it's a box with a 733Mhz Celeron in it, what do you expect? And XBMC is NOT a happy camper when it's CPU hits 100% at all times. It wigs out rightously. This would not be a problem on a more powerful box. And that's why my current desktop, an Athlon 64 that's being replaced in the summer, is being made into an XBMC box to replace the Xbox1 i currenty use