I use morale checks in my DF game. Yet, the last big fight by the PCs was nearly a fight to the death. The PCs inflicted unsustainable casualties on the Crazies, but they didn't break or rout. The PCs offered them a chance to, basically, surrender unconditionally (they didn't offer anything except "surrender or you'll die) but they didn't take them up on it.
I made rolls for them, but it was hard for them to fail.
- they were on their home ground. That's a bonus, right there.
- they had leaders in their midst. Another bonus. Plus, one of them was their inspirational berserk war leader, and the other was their chief, head priest, and lethal representative of heaven.
- they had nowhere to run. It was fight or surrender, and the costs for surrender were terrible - punishment from their priest and punishment in the afterlife for doing so.
- religion. The PCs basically stepped all over their religion, which upset them greatly.
They succeeded in their checks until automatic failure kicked in when they lost their center of gravity - their chief and their champion.
What could have made them fail a check sooner?
It would have been possible to reduce their roll.
- an avenue of escape. Had they had a place to run to, maybe they would have run. They were cornered in their homes in front of their families.
- take out the leaders. The PCs tried this, but had they managed it sooner, they could have ended the fight sooner.
- avoid the whole "attack their religion" thing. Hard, since that's what precipitated the fight. But once you've pulled the old "I am Lono!" trick, well, the backfire isn't pretty.
That's why the PCs couldn't crack the morale of the Crazies despite inflicting appalling casualties.