The effects are interesting. I'm trying to think of more historically accurate replacements, though.
How about Lake Guatavita in place of El Dorado. Fountain of Youth is a hard one.... Everglades? Something in Florida?
Well, I was thinking the Fayoum in Egypt, but I don't think Oases healed anything outside of Super Mario 64.
I'm thinking there's something in Iceland, but I haven't seen Bear Grills in awhile. There's also likely plenty of religious ones. I should probably look one up.
If they're in the disputed territory, they're not overpowered, they're an incentive for expansion. If you run into one by luck near your territory, then, yeah, it's overpowered.
The effects are interesting. I'm trying to think of more historically accurate replacements, though.