I feel like Necromancy is actually useful now, but not overpowered.
Sure, it can be a large amount of healing, but there are some limitations.
For one, it doesn't recover armor. This means it won't help at all until the character loses health, which likely won't be until all of at least one armor type is depleted. At that point, the Necromancy stacker will then be vulnerable to Crowd Control, and Necromancy won't be helping at all if they get CCd.
The biggest drawback in my opinion is that you're getting that healing instead of more damage, and doing damage is what makes necromancy heal in the first place. If a player has stacked heavily in vitality to have a greater window for Necromancy healing, it means they'll get even less healing because they're then doing even less damage than what a damage focused character would be doing.
Necromancy users are more likely to have longer fights (since they're doing less damage) and are then more likely to run out of armor and become vulnerable to CC.
My point is that it's a real tradeoff to invest points into it, which is the mark of a good skill in my opinion.
I did feel the comboing of Pain Reflection with Necromancy was pretty good though, especially on a tanky character.