1. Multiple attackers - if the attack is coordinated, you are facing the highest likelihood of all that you will be taken down. NO martial art prepares you for dealing effectively with committed multiple attackers, who in some cases have a pre-planned assault intended to put you down.
I've trained in a couple of tactical ground defense systems but bar far the best training I've seen for defense against multiple attackers while on the ground is the Systema ground work. It's based on their fundamental principle of constant movement. It is the only system I have seen that would really help you much if you were on the ground in the middle of a group of folks trying to stomp you.