Food+Production+Faith. Otherwise known as "having your cake and eating it too."
You always want to be growing your capitol as fast as your happiness will allow. But religion makes a huge impact in the game. So you want as much faith as you can get. Especially on the higher difficulty levels where you run the risk of not getting a religion. Or not getting the beliefs you need.
Every other +faith pantheon requires that you work inferior food tiles. Silver, gold, gems, pearls, tundra, wine/incense. But, Desert Folklore with flood plains means you can just work farms and are guaranteed to get a religion. No tough decision on which tile to work. And if you do decide to work a hills mine, you get faith out of it.
And then there's Petra. Petra isn't impossible to get on Diety. Especially if you beeline Currency. And Petra is game-changing. Every non-flood-plains tile now gives additional food and production. It's the equivalent of a Hydro Plant + the Hanging Gardens. Even without the free caravan and extra trade route, it's amazing, but if you add those? Boom! Oh and culture later on. And probably a food and faith-producing oil well! And if you're a civ with a pre-disposition towards desert starts, you get other bonuses on top of it.
Is it OP? Somewhat, yes. At least compared to other starts. With the exception of 3+ Salt tile starts, nothing is better. Personally I think Salt-rich starts are slightly better, because you get food + production early, which I value more highly than food + faith.
It's also about # of techs needed to work the tiles. Jungle/Forest starts inevitably require 4-5 techs to work the tiles: Calendar, Mining, Animal Husbandry, Trapping & Masonry. Desert starts usually require 2-3 (Calendar/Mining/Masonry)... and you start getting faith without any of those.
You still need to work 2 tiles to get a lux and a farm though. Salt starts *only* require Mining to work your +food, +happiness, +production tiles. You build one mine and you've boosted happiness, food, gold and production. So you can steal a worker and get rocking almost immediately. :-D
But no faith.
And faith is so valuable that even a tundra start is better than most starts. And there will likely be oil up in that tundra or nearby snow.
It's all relative. Desert is awesome, salt is awesome, grassland river with mines is great, plains is ok, tundra is ok, jungle sucks, and sea starts are the worst. Having to buy a work boat (the equivalent of a worker) for each tile? BS! Such a slow start. :-(