I think one of the biggest malus for observatories that nobody seems to have aknowledge now is that, since you get adjacency bonuses from farms and plantations, you will have more difficulties to have them because plantations and farms needs to be in your empire. I mean, lots of time, I had my adjacency bonuses from mountains and reefs that were out of my empire; here, with the Mayas, you'll have to build your plantations and farms first (so have the surrounding in your empire) to have the adjacency bonus. Maybe plantations and farms from neighboring empires, but in the beginning, without them, plantation resources will not give any adjacency bonus. So, yeah, having the adjacency based on improvement is somewhat a big malus (but since builders are the first thing you'l build in your cities for the farm housing, you might too enjoy the builder to build plantations).