You should have a damned good time with the customization since I think they nailed it in this game.
With the Challenge mode stuff seems like you only get GP and no parts are dropped. Just went through full session of it. Not sure how much it actually changes things up. Its nice to see the Gunpla Cafe stage back, minus the weird cast iron cake molds on the side lol
Im sad that the Event MC from 2 didnt make her way back into 3. D: Same with the VA Kayano Ai.
https://www.youtube.com/watch?v=SXSMF9bEOjE
One thing though is the Challenge mode isnt like how it was in 2 it seems. As it just tosses you into a field then gives out random objectives. Mine were all simple bouncing back between survive and destroy.