Hp will scale alot faster in legion then ever before. Even if you would crit for a million that heal would still only heal for roughly 30ish % of the nonbear tanks hp and 50ish% of dpsers and healers. Ilvl 850 dpsers and healers will have rougly 2million hp.
Damage patterns are slower instead of instant killing players they will get severly hurt and waste alot of mana on healers. A full durration enveloping mist for me heals for average arround 600k per cast.
Vivify will be casted when there is 3 targets that are hurt. Font also gets worse if raid is to spread its only 25yrd range afterall.
I have tested both sotc and mistwrap in raid. Personally i prefait mistwrap.