I think it's a reasonable assertion.
At the very least, it would be dominant when a traveller is within Shar's divine realm on the Plane of Shadow (as well as Shadow Weave enchantments, illusions, necromantic spells, and spells with the darkness descriptor being automatically Extended and Maximised).
Keep in mind also that Faerun's Plane of Shadow is mildly evil-aligned, and that all light spells are impeded there. Your shadow descriptor spells are also 10% more "real". Within Shar's realm, evocation and transmutation spells are also impeded, and light spells do not function at all.