Knights usually have a larger army than elves, but the quality is worse (although compared to the undead, it is much better).

I can't agree with the statement that the undead army has poor quality. The issue isn't the quality of the units, but the hero's skills. Without the "Bless" spell, what could the elves even do? Or the runes of the dwarves? Or the "Logistics" skill of humans? The problem is that the racial ability of a Necromancer – necromancy – has fewer drawbacks compared to the racial abilities of other heroes. Of course, you can use mentorship to develop this skill, which will provide more benefits, but this is not always achievable. And the claim that the undead units are bad is nonsense. Take liches or wraiths, for example – some of the most expensive units. The others compensate for their "low" quality with their cost and growth rate. But overall, the units of the Necropolis are solid mid-range units.