The treasure chest system is hank, at the beginning when I first started I got gold, rainbow, and silver chests easily as I ranked up. Now I'm at max rank, have above 100 win streak and yet I'm getting consistent low level wooden and copper chest that only contain money...
When you're your that high up you should be getting higher level items more often but it's as if the game doesn't want to give me them and it's irritating...
If you already got all of the unlockables for your main characters then there's really no point in going back to that mode other than getting extra money l0l