The MMEL allows you to operate with some things inoperative, that otherwise could impact on the airworthiness of the aircraft. Anything not mentioned in the MMEL that affects the airworthiness of the aircraft (ie part of the certification) or that is required by Operational regulations, must be serviceable. That just leaves non mandatory optional equipment and stuff not affecting the airworthiness of the aircraft such as cosmetic or comfort items which could be U/S without being specifically mentioned in the MMEL.
Certainly accumulators and fuel pumps must be servicable unless allowed in the MMEL, same for anything else to do with engines, flight controls, rotors, transmission etc