When we debate what Civ, strategy, unit, and or whatever is best or worse we should always say what settings we play with. There is a huge difference between playing a small map on normal or faster speed than playing on a huge map on Marathon. It's practically a different game. Play the game how you have the most fun, but we should all say how we play when debating things.
Amen brother.
Marathon is the only speed setting I play on now. Faster settings makes Civ 6 more of a quick turn based FPS.
Marathon, as you say, is a different game from faster speeds.
I use Large map, with 16 civs. That suits me. Huge map works too.
I am a VERY good player, and since the last few updates and DLC, I NEVER win easy on Immortal. I NEVER gets so far ahead that AI doesnt matter.
In fact, 1/3 games, I get destroyed pretty early in the game.
One MINOR detail, that only Marathon will show, is that if you choose to explore with your first warrior, and the unit is not close to your city......and another Civ, finds you. They will consider, hmm...an unguarded city. And take it from you. OR, build a few troops, then come for you.....if you dont grow......
Stuff like that doesnt happen in normal speed games, because building troops takes no time.