Venril is a broken encounter and can fail even when you do it right. That's the problem.
Well, that and they changed the script 3 or 4 times and broke it a few times, wasting months of peoples lives.
So if they're going to create scripted encounters, tune it in beta and STOP F'ing with it once it goes live.
If you follow the script to the letter, you should win. That's not the case for VS... now, we've taken VS down in less than 6 minutes before...and beaten him many times, but it still requires the "Luck" of not having the script bug out.
P.S.
I call BS on WoW's raid encounters.. my son plays a raiding hunter in WoW and I (Or he) have never seen anything in WoW that complicated or convoluted. (As Venril)
Nexona event? Perfection... loved it.
Oh, and in support of the original posters complaint, Venril being an unfair roadblock for some classes? Yes... Templars have to kill a heroic mob (Drussela Sathir) as one of their Mythical updates... that's pretyt lopsided.
Defilers get the OPTION of Venril or Overking. That should be for everyone.