Completing 2 challenges during a campaign mission awards an unlock. Are there any rewards for completing more than 2 challenges?
After completing more than 2 but less than 5 challenges, a second unlock will be revealed - while the first is a character perk, the second is a weapon. The weapon will be unlocked after 5 challenges have been completed. If you complete 5 challenges on your first run you will get both unlocks, otherwise you can replay the mission later to get the unlocks you've missed.