I'd definitely put Mag Warden ahead of Mag Necromancer, especially if the current PTS patch goes live mostly as-is. That said, I don't think it's all that great in an offensive role. There are some players that are really good about avoiding or blocking your Shalks, which really isn't all that hard to do, and that will basically cripple your ability to put any meaningful damage on that player.

For support, Magicka Warden is actually quite good. That's true for both a dedicated healer role, or as off-healing on a primarily offense oriented build. Warden isn't without its frustrations in this regard, though. The "Spiderman Heal" has been nerfed since I was actively playing the class, and is also completely unusable without either a friendly player to target, or your own pet bear (which is quite bad/unreliable outside of dueling).

If you're running coordinated premade groups in BGs or Cyrodiil, this shouldn't be a problem (other than the bugs/lag in Cyrodiil sometimes teleporting you backwards, into the midst of a pursuing zerg). But I do virtually all of my PvP by solo queueing Battlegrounds, where this can sometimes be a pretty major drawback to the skill. I'm also not a huge fan of the "cone" heal, especially when a friendly is at nearly point blank range and moving around a lot. Living Trellis spam can work as a substitute for off healing, especially since it's really cheap, but can't counter big burst damage nearly as well.

The main reason I switched away from Magicka Warden, after playing it as a main from Morrowind's early access through to Elsweyr, is that the offense is oftentimes too unreliable (and therefore frustrating to utilize). 'Course, I switched to a Magicka Necromancer, which really just means that I'm dealing with even worse versions of all the same issues.