Using NIST password guidelines as a compensating control has been accepted by every assessor I’ve dealt with (even the really bad ones). A compensating control must exceed the requirements of the control its compensating for, and the NIST rules clearly do. I’d say it has much more to do with how you write your compensating control worksheet rather than anything else. If you assessor is refusing to accept compensating controls, you should report them to the SSC, and then find a new assessor.