I think the key difference is Cordelia was aware that Chrom didn't notice her in that way, whereas Faye was not aware Alm had "friendzoned" her. That makes Faye more potentially abusive than Cordelia, but idk if I'd consider her a yandere? If Faye was truly yandere, she'd have harassed Celica...
I don't like tsunderes very much because the purpose of the trope is to fake a character development arc that is completely dependent on them falling in love. It makes whatever depth you grant the character completely pointless when their entire purpose is reducing them into being a love...