I think, as Frogman has said, massed anything is demanding, both because you need the ability to separate out the parts and follow them, and because there's often a lot of dynamic demands. But, these only cover certain attributes. I find that getting piano right is often tough for many systems to do- there is a certain weight and tone to the lower registers, and there is a lot of harmonic information, particularly in the upper registers, that gets flattened out in recordings. Close miking doesn't really improve things either, and it's not natural sounding. Obviously, a lot has to do with the recording itself. I used to use the Ave Maria from the Mission to hear how well a system did on staging and separation of individual voices.
