you acctually get mega units mostly if you're falling behind. Say you're 8th and theres like 0:30 seconds on the clock and you get the El tigre. He's supposed to help you catch up in rough times in the match
I don't know how much you have played but there is a modifier once you get to royal world where everyone starts with a mega unit. I'm wondering if it's based on the current megas or if you can get any mega you have ever had. I guess we will know when the next season drops👍