as he was not actually possessed by a great wyrm, but by a Primal - remember, the Bahamut in Dalamud that laid waste to everything in the Calamity was not the real Bahamut as it turned out, but a Primal 'shade' Tiamat created in her grief after the real one was killed by the Allag. Hence Nael was simply tempered as a Primal thrall just like any other, and the 'glowing red lines on the face' I wouldn't read into too much - they're just there for the coolness factor and probably also to help emphasize how inhuman, uh, inelezen, Nael had become now.
Having said that, it is clear the great wyrms can influence mankind - this seemed to be especially the 'curse' that doomed most, if not all, Azure DRGs eventually, this was shown even back during the level 50 DRG quest (the original one) where Nidhogg seized control of Estinien even then, making his later appearance in the storyline "in control of himself" all the more ridiculous as it came across as a handwave. Or foreshadowing, depending on your point of view.
Personally though, I think the main crucial factor to a great wyrm enthralling a spoken is if said spoken is of Isghardian birth, that is, they bare Ratatoskr's 'taint'.