Yeah, death heralds by definition should be the strongest (death heralds - you can tell just from the name), and level 6 for Knights is also possible. But what to do with dragons? Without them, Necros look kind of unusual!

As for the dragons, they've faded into oblivion; it's even written in the description how difficult they are to create. In a worst-case scenario, you could make a Ghost Dragon summon spell from Dark Magic (Incorporeal (like a ghost), 50% damage absorption. Applies a random curse on hit)
And besides, there are already plenty of dragons. Although you're right, but something has to be sacrificed, right? In principle, many options can be thought up with dragons; for example, put them among the neutrals.