I just noticed that the mystery boxes cost 20 gems in this game. In SvsZ2 they only cost 10 and weren't worth purchasing in that game. Someone kept statistics on over 800 mystery boxes he had collected and only about 1% produced a golden champion. For me less than 1% of the mystery boxes produced a golden champion.
Has anyone been lucky enough to get anything good from a mystery box other than the golden champion guard you automatically get from the mystery box after completing wave 3?