hm, from what I've been taking from the sequence quantum physics seems to apply fully at all levels, and the idea of it working differently/not applying is simply a matter of scale. For example an event causing a "split" affecting significantly macro objects almost entirely decohere, but not perfectly avoiding any kind of hard cutoff. Large systems definitely appear to work differently when you look at them on a large scale, but.. that appearance or classical hallucination is just an emergent property of underlying quantum effects.

Saying the quantum mechanics itself breaks down.. does not fit with the mental picture of reality I've taken from this, reality as entirely locally computable and with higher level effects based entirely on the base level substrate behavior. I'd like you to clarify what you mean by "break down", and preferably how reality would choose where to draw any line between scales where quantum mechanics does and does not break down?

I have read quantum physics has issues with gravity, perhaps that is what you're referring to? If so, I'd be interested in recommended further reading.