In general there are few/limited ways to get souls and few things to use them.
Limitation: Souls tome is the only "steady" way to get them. None to few buildings produce souls.
Feedback
Would be nice if:
A) Some tomes gave buildings that produce them, Graveyard, Mausoleum, Crypts, etc.
B) More spells used souls, units (besides Necro ones), etc.
C) Demonic themed tomes used souls too.
Dark and Demons
All paths have one element, ex Chaos: Fire, Order: Spirit, but Dark is a weird mix of frost and fire, no "dark" element.
I think Dark is a weird mix of necro-deminic tomes.
Would be nice if Necromancer had variations, as Necro-plague focus on blight, sickness and plagues.
And Frost/Water had their own focused tomes.