If the general in the unit dies, then the unit disappears...

You only get notification if the general actually commanding the battle (the one with the gold star) dies I believe during the battle...

It is a risk using these powerful units... If the wrong guy dies, its game over for that general and is bodyguard...

If you did not actually use those units for any fight during the battle however, then that is a bit screwed up...

I have lost family members (generals) myself this way as they make up a large portion of my german horde's cavalry line...