you have to keep in mind that skeletal legion is designed to be cheap, it doesn't have ranged, only melee, it uses double the population of Militia (the most similar unit) it doesn't have the GH bonus of Gnomes, you need tons of units to be competitive, and you need more GH than other races, if you look at the growth of the race over time, technology doesn't benefit them that much so, And yes, it is very dependent on iron and gold.
And I haven't even mentioned his main strategy "Swarm of the Death" or his exclusive spell yet, but believe me, it's designed to be:
- Really Weak Vs Orcs
-Weak Vs Dwarfs
- Even Vs Humans
- Strong Vs Elf
- Demi God Vs Gnomes
The costs alone doesn't reflect the full picture at all, you must calculate it based on atk / def rather than single unit and check the costs over time to make a fair comparison, and see the deltas simulating technologies.
Oh! I almost forgot, to make more gold, Undeads needs more population, with more population more housing are required and... more food...
Also, I need to mention, Undeads have a penalty to casting magic, similar to orcs, but almost no magical defense