So just so some newer teams don't read this thread and think that mecanum wheels are completely useless at holding their own in a game like this, check out this video from 2014.
https://www.thebluealliance.com/match/2014arc_qm29
The bright red and yellow robot (us) were running the new vexpro mecanums (before the metal casters oh how we wish they had the metal insert like they do now) and we easily could hold our own in a pushing contest. The key with mecanums is that the center of gravity needs to be as low and as perfectly centered as possible.
So, do a lot of mecanum drives get pushed around, yes. BUT, if they are optimized, can they hold their own against the most pushy of robots, absolutely!
Just something to think about before teams blindly throw it out because they are under the assumption they can be pushed around with minimal effort.