As to the learning curve... I don't think Civ games are that hard to understand, because everything looks so simple. In Civ2 in particular, it didn't take me long to figure how to beat the game in Deity, for example. But, as gameplay get refined, or as my brain becoming slower (or both), there is still some things I couldn't have learn by myself, in Civ5 for example. (one of the most played game of my favorite youtuber) Civ6 however I quite much didn't need to know anything that I didn't already know about, still I can't beat Deity on a regular basis. (I have to be lucky + be able to take advantage of that luck, i.e. be more aware of everything AT ONCE) As to Civ3 and Civ4, I'm sure I ignore most of subtleties Civ5 could have. (like do the number of cities increase tech cost ? Same for Civ6 when I think about it, or any Civ to be frank)
So whereas Civ is usually easy to play, there is a number of hidden mechanics that makes it hard to master, unfortunately. So unless you read the boring manual or learn it by someone else, you will not know them. That makes me think that the game design is lacking somehow. There should be constant reminders of what does what, like lacking information in Civ4 on how much gold per turn your new city will cost you. (Never been addressed, even with mods strangely enough. I simply don't get it. Another obscure part of Civ4 is how slaving works. But I guess all programmers are geeks who are above those considerations. They are busy enough to create the game, there's simply a missing link between them and the user, or even someone to tell them to make a more understandable mechanic)
That is that I wish I can master the game just by myself. (like I did with Civ2, even if it became a bit tedious to play once fully "cracked", however : remember that the player was crippled in Deity, and I don't know if AIs really got advantages. So it was a matter of "decrippling" us the fastest possible with the appropriate techs, which needed only very aimed and basic knowledge)
Morality : too simple to master and it becomes tedious quickly (Civ2), or too many hidden/unclear mechanics and it becomes frustrating. A good middle is Civ6 I would say, in the spirit, however I find the map generator too capricious. (well, or too constant in terrible spawns)