Two more observations:
1) The Mercurians most definitely don't get the full +strength bonus from technologies, only from the ones you research AFTER you built the Mercurian Gate. I finished research a while ago and for example their angels are at 19 (17 + 2 holy) strength, while they should be at 35 (33 +2 holy). They get all the other benefits from prior research, makes no sense to exclude them from the +strength bonus, they are basically useless when spawned late into the game.
2) Every 10-15 games the AI wins a cultural victory way too early into the game, like between turns 100-200 or so (on slowest speed Marathon). I opened the worldbuilder and found out that this is primarily "caused" by the AI capturing enemy cities, but it can also happen with the ones they build themselves. In the pictures below you see the insane culture values of these cities, and it all seems to happen in a couple of turns. Then the culture skyrockets for no apparent reason, even turning NEGATIVE in the process. This is especially frustrating, because there is no way to compensate for it like with the Mercurian problem, it just happens and the game you just invested 200 turns into is gone.
EDIT: Just got this myself for the first time, playing as the Lanun. I had just defeated an enemy faction and captured their capital, then after a few turns the numbers went crazy. Poduction/Culture/Faith went all over the place, jumping from 5-6 digit positive to 5-6 digit negative values. I attached an additional screenshot, look at the citizen tooltip on the lower right hand side, they seem to be the true cause for all of this. Anybody else experienced this? I'm having a hard time to believe I'm the first one with this problem (clean install btw, not tinkered with in any way).
Hmm, one week without a reply, it seems I am indeed the only one with the problem
Could this even be caused by a bad installation? I mean, it's not there right from the get-go, and like I said, it only happens ever so often.