From my playing exp
PVE
You're right even though I mainly focus on build rune and equipment for mystic, not many map I can use them
but some map they still useful
PVP
I stop playing around 2 months ago (just come back few days), at that time only few character is useful, but I still manage to get top 50 every week with 2 mystic and 2 guardian 1 healer (only mystic have +15 rune other is kind of half-ass)
Right now I'm back, my rank go down to gold 1
Manage to climb back to diamond 1 with kind of same team
Don't know mystic is less useful right now, or the buff abit, but so far kind of alright
(Have been practice with friend who got expert rank in arena so far, manage to win team all (some of them around 60% chance))
Anyway if mystic will got buff in future, I gladly accept it