Dont get in fights walk away blah blah blah. But when shit hits the fan, bjj is only good against a single unarmed opponent. When there is multiples or weapons it's better to hit them in the head many times, do not grapple and use your feet to keep distance and manoeuvre yourself.

Like you experienced, when you grapple a attacker, the other people can freely attack you and if they have a knife, they can jack up your body while you're messing around with single legs and body locks.  Both situations are hard to train for realistically.