I voted for my favorite Necropolis faction.

And that was a mistake. You should vote for the strongest faction, not your favorite.

Mage, simply an excellent secondary ability.

This ability is overrated. Mage slightly expands functionality, but doesn't gain anything in terms of effectiveness. It will continue to cast three spells, especially considering that it will only max out a couple of schools of magic. I might be wrong; I don't remember what needs to be upgraded for omniscience. If all other schools are maxed out, the second point doesn't apply. But the first one remains.

Minor artifacts

Too expensive.

Decent troops

Actually, they are weak. And the mage's negative combat stats completely negate the desire to march invincible armies around the area. Neutral creatures are still manageable; they are mostly defeated with magic. But when you encounter a fighter, things get really bad.

Added 11 minutes later
And to stay on topic:

THERE ARE NO STRONG FACTIONS; EVERYTHING DEPENDS ON THE SITUATION