How to unlock a jammed cam lock
Basically apply more force.

They may break, depending how jammed they are - they are cheap zinc castings most of the time. The screws they connect to are usually steel so they are likely to survive, and you can get replacement cams (or join the "my knockdown furniture is missing a few cams" club, which is large and not exclusive.)

You might try the trick of tightening them slightly before going back to loosening them, sometimes that will help. 

If you have a ratchet or wrench that you can apply to your screwdriver you can get a lot more force via leverage (helpful, though things may break) without the impact. If the impact is your best tool to apply more force, then use it. Square shaft screwdrivers can have a wrench directly applied, screwdriver bits can be inserted in the appropriate size socket for use with a a ratchet.