I think it would be wise to consider altitude and speed restrictions into the trip fuel calculations.
For example a major airport like Newark KEWR has you descend to FL240 like 20min before you would actually need to (FLOSI4 arrival), which obviously has a major impact on fuel usage. The calculated trip fuel is therefore always off (often by more than a ton) since Simbrief seems to ignore this.
KEWR isn’t the only example, multiple other major airports have procedures like this as well (EDDF, EDDM, EGLL, LFPG, KIAD… just to name a few)
Yes I could just add extra fuel, but thats not how it should be from a legal and operational standpoint since the trip fuel should be enough to get you from A to B according to your planned route.