- Which faction is stronger? (Necropolis, Tower, or Conflux)

That's true, but if there are many low-level neutral creatures on the map, then Necropolis can compete.

This is only if you're aiming to acquire the "Cloak of the Necromancer," otherwise, it's basically hopeless (referring to using your own units).
P.S.: And one more point, if there are other Necropolises, then it's definitely an advantage.