The huge factor in your child's experience is which teacher they have. It's the same one from 6-12, then 13 to 16 (ish).
My son's was great, really got him (severely dyslexic, possible ASD). When he was ready for the next stage, the teacher he would have had retired and we moved him elsewhere as it was so important to us that we knew which teacher he'd get.
There were many children in the school with additional needs- adopted children, children with medical needs, really severe challenges. The school coped far better than most.
I think it's a good system for ASC, up to a point. There's a pattern to classroom life which children find comforting, but no school bells- it's quite a calm environment.
There are things that seem a bit weird, but nothing really troublesome in practice.
The Theosophy behind it is definitely dubious, but it's more a philosophy than a religion. Most teachers I met 'nodded' at it rather than being devotees. It doesn't drive classroom teaching. The families tended to be quite sceptical atheists, but with an interest in alternative therapies.