The simplest answer without getting in the weeds is the FMC is a planning tool and VNAV is simply one of those functions the pilots have to work with. There are several ways to make it work for the crew and since ATC is seldom delivered in a prefect world, the pilot has to be prepared to intervene in the FMS logic. VNAV has greatly improved over the years and whether it be a Honeywell or Smiths application, the newest "boxes" do a pretty good job. The pilot needs to be prepared to intervene when they see the airplane going high on the descent, i.e. UNABLE crossing restrictions and they can do that with Vertical Speed or Flight Level Change, both of which have added limitations and operator hazards awaiting the user. Boeing aircarft have a descent ARC which give a excellet graphic didplay on the ND that shows how your descent profle is working. Airbus has something simailar but I don't think it's as intuitive. It's proably a matter of what youre use to so no big deal. I would dare say that less than 10% of the descent profiles in todays world work without some sort of pilot intervention. Take all this with a grain of salt as its been a few years since I have actually done one of these in a real airplane.