Jujitsu is an awesome martial art. I think it's hands down the best to study if you want to physically defend yourself against untrained opponents. It doesn't matter how much bigger someone is if you can choke them to sleep with a triangle or render a limb useless with an arm bar or knee bar.
The problem with jujitsu, though, is that in most cases, it takes far more skill to catch someone in a hold than it does to prevent that hold. To that end, in MMA, where everyone has a rudimentary knowledge of how to defend against it, it is less effective. A wrestler with fairly basic submission defense training can dominate a far superior jujitsu practitioner.
That said, when someone is truly world-class at jujitsu, it can get ugly in a hurry for anyone who goes to the ground with them.