From the ICAO manual on CDO: https://cfapp.icao.int/tools/ATMiKIT/story_content/external_...
> 1.1.4.6 With the need to ensure that CDO does not compromise safety and capacity, it may not always be possible to fly fully optimized CDO. As well, it may be necessary to stop a descent and maintain level flight for separation or sequencing purposes. The aim should be, though, to maximize CDO to the extent possible, whilst not adversely affecting safety and/or capacity.
Obviously level flight near sea level is less fuel efficient than at cruising altitude.
Likewise, descending into a significant headwind, or avoiding adverse weather will require more fuel than the optimal.
I'm not saying that a continuous descent is actually guzzling gas like crazy. There's very many smart people that have been optimising fuel usage in planes for decades.
I just want to emphasise that while the ideal descent is very fuel efficient, not every descent is ideal.
And of course, it varies significantly based on the plane.
This paper shows that interestingly, some A330 models used equivalent, or even more, fuel on approach than takeoff. Although I speculate that's more due to very efficient takeoffs rather than inefficient approaches.