Yeah situational difficulty is a big factor. I just started a Deity challenge YNAEMP game as portugal with Spain enabled. Barely had room for one, bad capital city. Not even enough room for a second in Iberia.
Should be a much harder version of Diety haha! I'm already doing things I never do like an ancient era-assault on Madrid to free up Space. Then to Africa a guess but I'm way behind the normal benchmarks just from the scenario.
There's a series on youtube where filthyrobot defeats 5 deity AIs in a 1v5 perpetual war map haha.
It's really ridiculous but it just shows you how insanely bad of a start you would actually need for a game to not be winnable at all.
That's really the beauty of this game. Almost any start can be won if you play right ( against the AI ), but not that many people seem to manage it on deity and basically no one has ever played a perfect game of CIV V.
Here's the series if anyone wants to check it out, it's awesome:
https://www.youtube.com/watch?v=SQaYywetOLs