Going by the 1 trailer they have out so far that shows some in-game action it looks like it might just be 1 or 2 hits and the npc dies. Does not look like the "normal" warg rider npcs will be hard to kill. How this translates to the roaming war bands I have no clue.
*note that this is just my personal speculation*
If it is just 1 or 2 hits and the npc dies I do not see how much fun this could be nor how this could qualify as actual combat.
Raids and big group content might be the only real good part of mounted combat for a lot of people.
*once again note that this is just my personal speculation*