RYR-738:
The pilot is responsible for terrain clearance. The only time he is not is when he is being vectored.
I think that, whilst terrain seperation could be considered a shared responsibility whilst under radar vectors, it still remains the
ultimate responsibility of the PIC.
Hence the absolute requirement for pilots to maintain SA and advise ATC if any clearance is unacceptable, for any reason.
We can descend below MSA, IMC, if under 'positive radar control', but that does not absolve us from maintaining awareness of terrain and our overall position - we still carry the can if things go wrong!