I now have a good experience with Abysia (several MP games), here're my views :
HI : quite good, with Fire Res being a great power cause it allows Anathemants to fire astray without collateral damage
! They are somewhere between a human warrior and a Jotun one. But their morale is only so-so, so SoC or Fanaticism are really needed in support.
Forget about the LW though, they're too costly to make them good for a Bless strategy and their unbearable enc (14 !!!) makes them sitting ducks after 5 combat rounds..
Mages : I don't agree with the fact that WA are "useless" : they're cheaper than DB, better researchers, and have Astral Magic. This is a very serious point, because in early game, after getting Alt3, you have a pretty effective force with say 25 HI, 3 WA casting B.Ether*3, and a couple Anathemants for Fireballs and SoC.
WA are also your best Blood Hunters.
So DBs are mainly my Demon armies leaders, with WA doing housekeeping ...
Abysian strength lies in Blood, so Cons4 should be researched asap to get the (in)famous Soul Contract (use a Warlock with Armor of Blood and Brazen vessel). Then Blood Magic to 5 for Horde from Hell, second only to Ghost Riders in term of usefulness vs cost in the genre.
Abysia is stronger in "mid game", but you'll soon have a problem with Research unless you've found some indy Library... In this case go to Cons6, build Lanterns and focus on Blood only.
I agree with the proposal of a Scorpion Pretender, but I rather give him Fire2/Earth3/Air3. Fire isn't really needed, Earth gives Dw Hammer, Legion of Steel, Ironskin and Blade Wind, Air allows with Earth to make Elem Staff, Eyes of Prec,Quills, Staff of Storms, cast Seeking Arrows... very complementary to Abysia.
Overall Abysia is quite powerful but needs some "focus", more than say Man or Arco IMHO.
BoH theme is another beast : more Blood, less Fire, human troops...