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

You're forgetting about the first level - skeletons, which in skilled hands are unmatched on large maps!

When playing as Tower or Conflux, you can clear everything up to Crypts with 1st-level units, and do so without losing any units; whereas skeletons at the start, unless of course you take Galtran, plus on top of that at 160% you can buy a 2nd Hero and have nearly 100 skeletons on the 1st turn, and even then, skeletons are still slowly killed off. My point is that at the beginning of the game, 1st-level units matter very much not for Necros, but for others. Skeletons prove themselves later... For example, if you run around with Fairies or Gremlins (well, those are at least something) in your army in the 3rd month, I think they'll laugh at you, but that's exactly when skeletons already rule.