The big issue for me is that even if I imagine that they fix all the bugs, make the UI good and address some of the more obvious design flaws, I'm not seeing a good game. There are far too many things I don't like about it. I have 2000 hours in Civ5 and 2500 in Civ6, but I don't find Civ7 fun at all. It's tedium and boredom basically from the first moment. Slow, dull, uninspiring, annoying to play, full of very poor design choices, and I don't like the age transition at all.
They also completely abandoned the symbolic realism that the series was known for by letting any leader lead any civ. Even if Civ games were obviously never meant to represent an accurate portrayal of real-world history, they did have a certain symbolic realism which this new civ/leader mixing completely erases. There's no sense of identity, no real flavor. I don't feel some kind of familiarity towards leaders anymore. I used to have a fondness/rivalry/whatever with the various leaders in 5 and 6, but in 7 that's completely gone and I just associate them with a list of abilities. That's not to mention that so many of these leaders were never really leaders in history. Ada Lovelace to lead Britain? What? What even is that? They're just randomly selecting historical celebrities at this point. The symbolic realism of the Civ series did matter, it kept away the feeling of playing in an arbitrary fantasy universe and it made it fun when things in the game happened to actually coincide with the course of history. Abandoning it is not an improvement. It's just a case of pointlessly removing one of the charms of the game, and nothing whatsoever is gained in return. If they wanted us to be able to customize our "build," they could just have let us select abilities similarly to how the memento feature works.
On that note, I also hate mementos. What a terrible concept. You now have to play the same civs over and over in order to unlock power? If you don't, you're playing a scuffed version of the same leaders? It fiercely punishes trying different leaders and instead encourages just playing the same ones repeatedly, which is not a good design pattern. It also takes a steaming dump on players who prefer to play random, because not only will you absolutely never unlock all the mementos that way, you also can't really utilize them properly.
That's just a couple of examples of what I hate, and there's so much more. Too many to describe without writing a whole essay. All in all, I do not enjoy this game. There are so many design choices that I can't stand, and the general overall gameplay is also just boring and unimaginative. It's slow, sluggish, a chore to get through, and on the rare occasion that I'm able to force myself to play until the first age transition, I'm immediately hit by an overwhelming desire to abandon the game and do something else.