I would agree with runningdeer. You can't really put 'bilingual' schools into one basket. They vary greatly.
Our daughter moved from the Montessori system in Australia, from an excellent school, to a very well established Montessori primary school (bilingual) in Zurich, and we were very happy with her experience. The cost factor is phenomenal, but my husband and I are teachers, our children's tuition is bundled with the salary package and we are paying significantly less than the average family who use the private school system.
Anyway, she was 9 and her teacher basically had to invent a whole learning process for her because she had zero German, she was 4 years in the primary school here in Switzerland, and has this year gone into the local secondary school. We chose not to put her straight into gymnasium because even if she had managed to scrape in (her German and Mathematics are actually very good), the competitiveness for us was a big issue. We considered it better that she move to the secondary (she's in Sek A which is the highest academic level in the local school), get confident with being in the traditional school system, and see how it goes. Many of the children coming out of her Montessori school do go straight to Gymnasium (even on pure demographics you'd expect that), and a lot transfer to the 'kurzgymnasium' later if they didn't go direct.
There are so many tracks. In our case, we are totally happy with the Montessori approach at d'Insle to the language development, and to taking each child as an individual learner and keeping them on their own track, and at least in our case, at the primary school, there has been a very good consistency of staff (same teachers for 3-4 years), and many of the teachers are dual trained (Montessori and approved university teaching degrees).
I also totally agree with svaha's points, and I would always ask what the qualifications of the teachers are, whether the school complies with the local curriculum, and what the turnover is like.
In our case, our children got plenty of additional support in the Montessori setting for their individual learning needs. In the Sek, although she is coping (and getting better than average) marks in German, they know she has things that she needs to work on, so they offered her two additional sessions per week with the 'German as a second language' class. She skips one class of biology and the typing class to do these intensive language sessions, and we are happy with that decision. She is able to catch up on the biology very easily (she has an awesome grounding in that from Montessori), and the typing is no problem, as she can easily practice at home...
Anyway, that's a bit of a tangent. There are many layers to the issue.
Oh, and update, from friends who brought their 12 year old to Switzerland 4years ago, he did 2 years at the 'RealSchule' - which is the lowest level of the secondary, because that is where the intensive German support teachers are to be found. They loved him because he was smart, polite and very motivated (most of the kids in "Real" have learning difficulties).. The school offered to transfer him to the general secondary level after 12 months, but the parents found out that he would get no additional language help, so they kept him in Real for another year. Then they put him into a private German secondary school for a year, and he just found out that she has a transfer to the Gymnasium. If I understood correctly, he's done 4 years of 'grounding' in German schooling, and now he's going to do a 3 year Gymnasium transit, and from there we hope the 'world is his oyster'...
So, it can be done. Navigating the system is not easy, but I feel that at least here they promote all the layers and different ways to access tertiary education, and don't just stream out their 5% and tell everyone else their life is going to be a failure because they didn't get straight into university (which is my experience from Australia)...