I did this fight (25-man) using no bar mods other than CT.
I had 44 hit rating, owing that I collect more crit/Spirit Spellpower gear for my build, and much of it does have a bit of hit. I MC'd each mob in turn, though the tank actually managed to kill the fourth by accident after I'd Widow Embraced the third.
I never ran into any of the problems being listed by Priests, the abilities were always available, and the mobs were never immune (and they were in Consecrations). I don't think our main tank took a single Frenzied hit.
What might be causing Priest MC to break early is the Poison Bolt Volley. The priest taking damage, from what I can tell, can easily cause it to break. I've countered this by making certain that one healer watches me and cleanses the poison ASAP. I MC about five seconds before Enrage cools down, and I hit Widow's Embrace the moment she enrages. Never failed.