Necro tank is ok, but behind DK and Warden based on my experience because:
1. Necro's "pull" ability is hot garbage. Not only can you not control the pull, but you can also pull enemies OUT of the aoe. Hard pass.
2. The root ability has a 2 second delay. Not the end of the world, but not nearly as easy to manage as the instant cast on DK talons and Warden gripping shards.
3. The goliath ultimate (or whatever the tank ult is called) is a bad version of Molten Armor since it doesn't improve mitigation or help the team in any way.
4. Pro: Ultimate generation is really good on the necro, making it a strong offtank choice.