They look really really strong if they get a couple desert cities. They will be swimming in gold like Scrooge McDuck.
They won't be nearly as strong on non-desert land and will be weak early on with the production penalty.
Looks like a fun civ. They may be my 2nd new civ to play after the Moari.
They tie in really nice together:
-If you start with all desert tiles, your city naturally gets +6 food and +6 faith. That food, combined with the little extra from granary/water mill/etc... is enough to get you to size 4. As Mansa, I don't think I ever want to grow a city past size 4.
-Assuming you get desert folklore, if you have a city centre-CH-Holy site triangle around all desert with the CH on a river, that's a +5 CH, a +7 holy site. You never have to worry about either of them meeting the minimum requirements to run the policy cards
-International traders get +1 gold from flat desert tiles. Which means early game, when your trader usually only gets 3 gold from a route to a CS, will now be getting more like +7 or +8 gold. That's a lot of gold early
-However, it means they don't run internal trade routes, meaning they don't get the production bonuses from there.
Other important parts of their strategy:
-Mansa should never build an industrial zone. It's completely useless to him
-He should only ever have to build 1 encampment, as all units can be purchased from there as necessary
-He'll have enough money to buy whatever GP he needs, which as mentioned, give bonus era score. They definitely want to run golden ages since traders are very valuable
-They are definitely hurt by the change to commercial city-states splitting their bonuses between commerce hubs and harbors. They synergize with a lot of city-states too - all of the trade ones, and even others like Hong Kong (since they don't build other things, I can imagine running a lot of projects) and all the ones that give UI (since they need something to throw down on desert tiles).
He also gets the edge that nobody else likes desert to settle in, but he can settle anywhere in the desert he wants, since he probably doesn't really care if his cities grow. Even a size-1 desert city is going to be valuable to him for the faith and trade routes alone.