I have finally played Joan for the first time last night. Yes, his fast settler & worker is pretty amazing for fast REXing. However, I have some complains:
1) On high level, you can't REX too much without wrecking your econ. 1st of REXing is typically limited to 4 cities max before code of law. Since this is the case, other leaders can match Joan as well. The only exception being choke point on map like Achieo...
2) I still don't like his UU and UB. Oversea colony before astronomy kill your econ big time due to no sea trade yet. Since this is the case, why not wait till Astronomy for oversea REXing then?
3) Someone mentioned that Colossus combined with Joan's UB is a killer combination for water tile. Yeah, that is true, but with no sea trade before astronomy, I can't reap the benefit of sea trade, making astronomy a priority, which is again, conflicting with Colossus. Given that his UB (I can't pronounce the thingy) need education & banking, which is pretty late, I think further delay of astronomy to prolong Colossus lifespan isn't feasible.
In short, what I am trying to say is, it is very difficult to leaverage Joan's traits, UB & UU together and come out with a killer strategy, unlike Julies Caser's organized&imperialistic-driven prat rush, Gilga's zilga&espinoage-driven vulture rush, or Cyrus financial&organized-driven immortal rush, etc. Joan's UB & UU & traits basically called for different emphasis on tech pursuit.