Is UnityAds the only ad network supported or can I use others when I develop on the Unity platform?
You’re of course welcome to use other advertising networks for your Unity games, but we hope you find that Unity Ads actually brings you most value 
E.g. a search for “unity games monetization” using your favorite search engine, should provide you with some links to other ad networks.
Hope this helps, and good luck with your game.
/Rasmus