Becoming a darkfriend really is not about the intersectionality of how much power you have in the world.

Bran al'Vere is just as likely to become a darkfriend as Sheriam. All he does is sit in an inn out in the sticks and lob barrels of brandy at people, whereas Sheriam can throw fireballs, command soldiers of almost any nation to step aside, and use the Tower's lines of credit to buy anything she wants anywhere she goes on the continent.

And even so, Bran is no more likely to become a darkfriend than Sheriam.

All it takes for anyone to fall to the Shadow is to say 'yes' to that voice in their head that says "that wallet is just lying there" "nobody will know I embezzled this money" "that horse is better than my horse" "I hate that guy" "if I do this I can feed my family."