I interviewed a girl who had been to a Steiner school for a place on a degree course quite recently. She was bright, enthusiastic, and highly motivated; she had done loads of interesting things at school, including writing directing plays. She was well suited to a place on an English degree (the course I was interviewing for).
However, her exam profile was very poor, and she was applying as a mature student (ie over 21), because she would not have been able to get a place on a degree course at 18. She's doing quite well now, and will complete a degree by the time she is in her mid-twenties.
That's all well and good. It seems clear that this school provided lots of creative opportunities. But what about if your bent is not arts, but sciences? What about if what you are really talented at computer programming? I can't see, from talking to this girl, that her school (though she'd clearly had a good time there)' that the school would have served their needs.
And the lack of formal exams (whatever one may think about the uselessness of exams in principle) did disadvantage her. It was lucky for her that she came to the place I teach, which has a mission to offer higher eduction to adults who may not have formal qualifications. Otherwise she'd have been stuffed.