In arenas this is the problem with Assassination. This needs to be addressed before 10.2 comes out. Deathmark needs to be undispellable in arenas.
All these changes for 10.2 are excellent but none of it is going to matter when everyone class abilities and racial just removing our dots/2-minute cd Deathmark. None of the changes will even matter.
I was about to make a post about this. Or if you dispel the bleeds with Deathmark you get hit for all its remaining damage
Up to maybe 1800 it’s np. But once you start pushing 1800 to 2100 people get the cue from their addon and dispel it instantly. Always thought this was really stupid to let a spec’s biggest CD just be insta dispelled like it’s nothing and as a result most top players just say F it and reroll Sub.
Just make sure both players don’t have trinket or any defensives, stun them both simultaneously for 16 seconds and you’re good to go! 
I dont think ive won a single game where i finished someone off with deathmark. Its almost a waste of a press.