I think the Four Fiends are decently written. Like, what we see of their past in the side quest is really good and lets us get an idea of their motives and what life was like during the Contramemoria (Its just a sin that all that is hidden in a side quest and not incorporated into the MSQ itself.) The writers just decided that they where not going to be long-term characters in whatever they have lined up. It sucks when that happens to characters you like (I still miss you Moenbryda...) but it happens. I wonder if the complaints about the Four Fiends would be the same if they where unique FFXIV creations, or if some are putting them on pedestals just because of their legacy.