It's not difficult at all to unearth the Grail if you have some skill. And you can find the excavation site after discovering one obelisk. And even without discovering the obelisks! Experienced players will understand what I'm talking about, and for the rest, let it be a small puzzle :) I try to visit the obelisks myself, and when the game reaches the 6th week, everyone goes "potato hunting." I've dug it up 3-4 times in my experience (for statistics - within the range [500; 1000]), so I don't see any reason for restrictions, especially based on the type of city.
I'm against prohibiting the installation of the Grail in a Conflux or Tower. If these cities are starting cities and the army comes from these cities, then why can't I use the Grail's bonus? But the timing for unearthing it could be made more flexible; 113 or 114 might be too much. And then - we need to experiment, in special tournaments, for example.