Wights are not a really good example .
By the time you can summon them with the lvl 9 spell most of your opponents should be able to get wither bones casters .
Wither bones kills the wights really so quickly that it is not funny . Especially when cast by lamia queens or Sauromancers .The former is available to almost any nation . Eagle Eyes , Wither Bones x4 , and 1 Relief casting Lamia or Ivy King and your Wights are gone really fast .
Mech Men and the like otoh are very vulnerable to spells that target MR like nether darts etc. .
Imo the only really a bit overpowered summons are the blood summons . FoDs and Devils are uber . And the Devil is strangely the cheapest summon also . Vs the Devil only Air and Cold spells work but holy pyre and the other fire spells don't . Really problematic it gets with soul contract generated devils and with wished blood .
I think there is nothing more cost effective you can do in dominions than clamhoard and then wish for blood . Then you get decent Thugs with the devil leaders from horde to hell , good defense troops with vampires and vampire lords and good + cheap troops with the devil .
With national troops imo unfortunately the supply system is a bit broken . In SP i always play with 500% or 1000% supply multiplikator and 200% resource multiplikator .
The resources are the second unfair thing , ulm and other resource intensive nations have big problems getting lots of troops .
But some national troops that require few resources are really good also , especially the centaur warriors from pangenea and all kinds of missile units with flaming weapons .
Basically Dominions is quite balanced but because of the too serious supply + resource restrictions only resourcecheap nationals are worth being recruited . Zens spell mod does it just right , ghost riders need 6 death instead of 4 to be cast , same for wrathful skies .
If you add a supply x300-500% multiplicator then everything is fine and most national troops are worth being used also .
I always play SP with 15 or 16 impossible AIs , 200% resource multiplicator and 1000% supply multiplicator on a rather small map like karan . It is fun and very often i lose to the ai hordes
.
Even with caelum and either doing false horror spamming or thunderstrike squads i got overwhelmed until turn 40 by 3 attacking AIs .