Yes, they take a significant amount of extra fuel, but that significant amount is very carefully calculated because carrying more than legally required adds up to a loss quickly, especially if your competitors are sharper at calculating these figures.
The are ~100 parameters that go into these calculations, ranging from almanacs with tables of various trade winds and densities to engine types, consumption at various altitudes and power settings and so on. And then the required hold time, first alternate, second alternate and legal reserve required on landing.
Getting that code certified still makes me break out in sweat, but good thing that this kind of thing is reviewed very strictly. That also makes me wonder how wise it was to have the software this article is about as a SaaS rather than something more integrated and less easy to mess up using an update. Maybe doing it as a SaaS is a kind of loophole that avoids certification? (not current on this stuff).