Depends, in some phones such as mine. the default partition is efs whereas CM uses ext4. so if you flash the stock ROM directly on CM, it will go into a bootloop. Plus, in a nandroid backup, your settings and data gets preserved too.
I had that too. But then u can use Hermdal for CyanogenMod and Odin for Darky ROM and get back to working state.
I had that too. But then u can use Hermdal for CyanogenMod and Odin for Darky ROM and get back to working state.