I upgraded to Premium yesterday. It is working fine for some MKV files but I just have a few that won't play. I checked in VLC and the subtitle code is SubStation Alpha subtitles (ssa)
For now I'm using Chrome to play with VLC and cast that tab, but doesn't allow one to use a playlist, which is not so great when ill and can't move much around, sniff.
EDIT: Nope, can't cast it like that either apparently since I upgraded to Premium, just looks like a gray blur.