I'm not sure that lowering AC and resists is something you want a group HoT to do in most situations. Aside from possibly a few instances (I can't think of any honestly but there has to be something out there) your group is taking damage from either an AE or phantom strike/erratic strike-like mechanics. Healing them and making them take more damage from the thing that caused the need for the heal seems counterproductive and would force, at best, a stalemate, or (unless you're using it after combat, in which case the drawbacks are trivialized) more likely exacerbating the problem. EDIT: I'd like to add that most pets are pretty much group members, some of them (mage pets I hear? idk I'm a rogue) already have hefty mitigation/survivability issues. This would run a huge risk of obliterating them.
That being said I hate the shaman group hot and I banished thade to caster group when he got it. I don't know if there's a better way to do things, but it seemed to mitigate the drawbacks the most by doing this since the slow didn't affect casters and the snare, for the most part, wasn't a problem since they had to stand still to cast (unless they were sneaky-casting). If they had to move, they were fully capable of getting a heal-tic while they were casting/greyed out, clicking it off, and heading for the hills/clicking murk beads/getting a group heal.
My understanding is that the Runics were, either completely or in part, intended to be niche-spells. This one's obviously aimed at healing the caster group (or potentially a tank group full of knights) in a raid. Not viable in some group set-ups, not vialbe in some encounters, but useful in several situations.
