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

Yeah, well...
I think that master gremlins are strong at the beginning, but when the enemy reaches them, their strength somehow decreases. Magic can help here, of course.
It's difficult for the undead to play at all without resources. Otherwise, you can either upgrade vampires or build liches. If you can't find stones, crystals, or sulfur, then you'll have to stick to levels 1-3.

Yeah... but who will let the enemy REACH the master gremlins? :D I, for example, don't allow this in 95% of cases... and if someone does reach them, it's in such a small number that it's not even worth talking about (of course, I'm talking about the beginning of the game - when there are no mages or titans).
And the undead are weak - level 1-3 is practically useless here... at the beginning of the game. Skeletons are strong in large numbers... there's no such thing here. Zombies are just meat... slow and stupid... it's a horror. I don't consider them as a unit. I don't build ghosts in 90% of cases at all!!! So, see - what a fun start! If there are no vampires, then everything, development is terribly slow.