Bjj is extremely street effective. The fact is that if you know how to fight on the ground, and the other guy does not, you have an enormous advantage. Size doesn't really matter if the other guy doesn't know how to grapple. I know guys from my gym that weigh 130 pounds that can take down untrained 200 pound men without breaking a sweat. Unless the dude knows how to sprawl or guillotine, he's probably goin' down no matter what the weight difference is. Not to mention the fact that most guys think that street fights are about punching the other guy in the face and nothing else. Untrained guys don't expect a takedown and that's why they are so easy to land. If you have a good takedown, he probably wont be able to hit you at all before you get him to the ground. Once on the ground, you'll know what to do. He'll give you so many ****ing openings it will be like shooting fish in a barrel.
I've trained bjj for 3 years now and have had to use my skills twice during this time. Once I got into it with some jerk at a party. I said something he didn't like so he threw a beer bottle at me and charged. I got up and he did the overhand right with a duck and weave and I grabbed his head and guillotined him till he went to sleep. Second time I was cheap shotted and instinctively went for a double leg takedown, went immediately to side mount, mounted, when he tried to push me off I arm barred him. He was screaming. I asked if he'd had enough. He said yes. I let him go. The thing about bjj is that when you learn the submissions, you also learn to control them because you don't go all out when you get an armbar on one of you classmates. You get the move in place and slowly apply pressure till they tap. The difference with real life is that you apply pressure and in stead of waiting for the tapout, you ask if they've had enough. Another nice feature of Bjj is that, depending on which school you go to, you will probably be rolling 30-40% of the time. This is excellent for your confidence. You learn to use the moves in realtime. They are battle tested.
It is true that if you are fighting more than one person, Bjj shouldn't be your first line of defense. You can mount a guy all day but it isn't gonna do you any good if his buddy stomp kicks you in the back of the head. Some schools go over judo throws and one of those might knock out of the first guy, but if there are more than 2, you are probably screwed. As far as singular martial arts go, I'd say bjj and judo are the most street effective in 1 on 1 situations. Pair it with some striking and you will be a force to be reckoned with.