CW's problem is that their troops, while fairly effective, almost all have very low protection and defense, and no shields. This means that they usually take high casualties, especially against archers and battle mages.
On the other hand - given enough numbers, they do handle things like knights and heavy cavalry fairly well; probably 2 to 4 carrion critters dies for each knight, which compares well to most normal units.
They do generate enough carrion dogs and other high movement creatures to serve as effective cavalry, especially with Puppet Mastery.
CW does take some ... management of the troops. Given that the first carrion critters to reach the enemy are going to die (especially against cavalry and such), you want to put your lesser carrion in the first wave - normal manikins, carrion chickens, carrion piglets, etc. This way they take the brunt of the initial losses, conserving the more valuable carrion.
Another way to spruce up the carrion bunnies is to add some vine men / vine ogres. Forge a Vine Crown and a Treelords staff and one dryad can summon 4 vine ogres for 1 nature gem. (Skip the staff and she can get ... 3 vine men? Or 2?) The nice thing about the vine walkers is that they don't have any upkeep and don't eat, just like your carrion, but they in effect dissipate the effect of banishment.
It also takes some management and creativity to gear up the gem income and research needed. Nature gems are best used for generating your carrion lords (who are best used to reanimate), rather than carrion ladies or centaurs. (The carrion chicks are okay for research, given that they have no upkeep, but there are better ways to go.)
So - since CW also has death mages and income, you want to ... generate undead mages. If you don't find sages or some other high value researchers to recruit (sparingly because CW has a cash flow problem), then revenants can be very cost effective, especially since a skull staff allows a revenant to summon more revenants. Another thing to do is use some of those death gems to build Skull Mentors - 10 death gems for an extra 9 research is pretty cost effective, even more so if you can get a dwarven hammer or two along the way.
Of course, to _get_ the dwarven hammer, and other decent forgings and summonings, I find you need to go with a fairly broad range of magic on the pretender - I like to have Earth 4 as well as Nature and Death at least 3, and preferably 1 or 2 more elements at 3 or higher. (Not the other sorceries - having to use a pretender to blood hunt isn't very effective, and Astral - well, Astral would be okay on an immortal Pretender.
)
The broad range of magic allows the pretender to find magic sites CW would otherwise miss, many of which give recruitable mages, and allows some of the mid to high level items and rituals to be handled by the pretender.
In addition, even without any magic on the pretender, CW can eventually get some decent mages via death and nature, and will need to : spectres and mound kings from death, Lamia Queens from nature, which will eventually be able to summon Couatls for Astral magic.
Along the way, CW definately does need a bit of luck in finding magic sites. It's definately not an overpowered theme, but it can be fun and can give some of the other nations a run for their divinity.