I agree that it’s hard to see the attraction of Yelmalio in-world. At least in RQG, Ernalda has better combat magic (this changes when he at least gets Shield). Are Yelmalio’s myths cooler? I don’t think so - Elmal Guards the Stead is victory through tenacity and sacrifice, while Hill of Gold is losing but surviving and maintaining that ”purity” by not having sex. Humakti geases tend to enforce honorable behavior; Yelmalio’s stop you from bathing and stuff like that (but then you can talk to birds!). And then there are the constant ”moral victories” (i.e. defeats), and it doesn’t help that the cult is so commonly used as someone’s else’s tool (and resurrected for just that purpose).
It’s hard for me at least to see the attraction. If I’m an Elmalite, they can keep their Yelmalio. My god is Orlanth’s loyal thane, and you want me to abandon Orlanth for a foreign god in the service of Yelm? Who are you, the Teller of Lies?