The question is how much of the difficulty is due to the AI’s starting advantages, and how much is due to its skill.
There’s no doubt that the AI is pretty good at a lot of things. It expands its territory really well in the early game, and is very good at picking off injured units. Going down in competence a bit, tactically it’s capable of using overwhelming force to win battles and conquer cities.
All of this is far ahead of the AIs for other relatively recent 4X games, so I don’t mean to be overly down with recent posts. It’s more that it’s interesting (and a bit disappointing) to be picking holes in the behaviour of a good AI. Far surpasses staring in bewlderment at an AI that doesn’t pose the slightest threat.
Diplomacy is a much more interesting design than in previous games and the AI seems fine at it, if occasionally hamstrung by random events.
Where I do feel the AI is losing out at the moment is in strategic concerns (plus ça change). It seems to build a reasonable military but doesn’t do a good job of building enough improvements (should it be making more workers?). This leaves it behind the player in the long term in both resources and science.
All of that means that an early war with an AI nation is a severe threat to the human, generally forcing some painful tribute payments to avoid it. Once the player has risen out of the starting hole, the game feels pretty much won though. That does require capturing territory though, so if you don’t have space to expand without going through an AI nation, that’s probably pretty difficult.
Starting position remains therefore a very big modifier to difficulty. My most recent game was on a random map, and I rolled an arid map full of mountain chains (though with a very large central sea, so I’m not sure it was arid plateau). As it happened, I had one neighbour who made things difficult early on, but importantly I had a huge backline area full of barbarians and tribes. I built a reasonable military then sent it all off conquering the minors, paying heavy tributes to my neighbours to avoid war and buttering them up wherever possible. By the time I’d conquered everything in the backlines, I had more cities than anyone else and a far better economy and was finally able to secure my other border and feel certain the game was mine.
But yeah, if I’d had another AI nearby taking two thirds of those backline tribal spots I would have been in a lot of difficulty.