Well, concerning Fallen Heroes: I think it’s because the Undead aren’t nearly as aggressive as the orcs typically. You’ll see armies of orcs rushing at villages pretty quickly, but the undead seem to bide their time more and play the waiting game before they start moving in. That’s my experience.
That said, Fallen Heroes also aren’t as deadly to me as Orc Warriors. The main strength of Fallen Heroes is they have no fatigue. They have good stats, but coupled with that they have absolutely no fatigue whatsoever. So, when you fight them, you need to kill them fast, before they can wear your Battle Brothers down by just having more energy than you.
And my armies tend to be very much focused around hitting the enemy hard and fast. As I said before, my favorite army setups involve two-handed weaponry in bulk. I can keep a couple tough guys in front (nimble tanks or shieldbearers) while having pikemen stab from the back, or while having a two-handed swordsman perform a wide sweep or cut before stepping back once the dudes adjacent to him are dead. They’ll wear out fairly quickly, but they’ll kill quickly, too; and I have a captain/commander behind them blowing his horn every turn to keep them hale and hearty.
So, for me, Fallen Warriors aren’t that much of a problem. At least, they aren’t a problem until you starting factoring in the enemy having a necromancer or two, ghosts, and overwhelming numbers. Those kind of fights are terrifying problems.
It is quite easy to get unlimited fatigue yourself. Just have two brothers with high resolve and take turns spamming the refill fatigue skill. Keep the other brothers in a circle around them and you can fight indefinitely.