Dark magic is stronger and should be stronger than summoning magic.

Is it because you want it to be, or are there more objective reasons?

Everyone should be equal, both in the late game and in the early rush.

That's funny. The balance lies in the fact that each race has its pros and cons. Necropolis has the advantage of being able to break through enemy lines due to their infinite mana (if you don't believe me, I can post some revealing replays).
If you're not interested in playing, I can suggest trying a different race. Also, there used to be debates about balance, which led to the creation of Tournament Edition; I suggest you try it (there, the balance has been refined by dozens of the best players and many tournaments). However, in that version, Necropolis is more nerfed than buffed.