I haven't quite quit, but I'm getting awfully close.
The game just feels tedious. Everything takes forever, and none of the decisions really seems to matter. I've started at least half-a-dozen games, got into some middle period, and quit out of boredom. (And, as background, I've been playing Civ since the Avalon Hill board game, and have bought and played every version near launch since Civ1. And I've played enough games of Civ6 to have victories with at least a third of the leaders, on different map types and sizes.)
It bothers me that there are entire subsytems that add micromanaging to no effect.
Religion
Perhaps the most enjoyable game I have played so far was as the Kongo, who are designed to ignore religion. It was liberating. I realized that unless I want to go for a religious victory from the start, I don't have to build holy cites, shrines, temples, apostles.... I can just ignore the whole thing. And it has almost no impact on how I play the rest of the game. The only thing you have to do is keep track of whether an AI civ is getting close to its own religious victory in case you have to kill them off.
This came after a game where I had started my own region and was spreading it around the region when three English missionaries showed up. I contacted Victoria and asked her not to convert my cities. She (of course) agreed. And, the very next turn, (of course) all three missionaries tried to convert one of my cities. So, I denounced her. And she used all three missionaries against my city the very next turn. So, I want to declare war, but apparently I have to wait for some arbitrary and unknown length of time to get a casus belli. So, I declare war anyway. Note that this is the first war I have declared in the game. And every other AI immediately decides that I'm a warmonger (which will, in practice, never go away). Really?
My conclusion: it is better to just ignore the entire religious subsystem. (How does that make the game more fun?)
Spying
Let's ignore the fact that the user interface is abysmal, requiring about three times as many clicks to do anything as should actually be necessary. And the fact that I have no idea how one gets more spies after the first one.
What does matter is that the "siphon funds" event is potentially game breaking. (Example: I played one game where I was ten turns from completing the final two parts of the Mars expedition. I get a notice that a spy has started siphoning funds in an amount at least ten times higher than my treasury. Units start getting disbanded every turn. And, since I have a large empire and am at the limit of what I need for amenities, I'm running a policy where garrisons add one. As units disband, rebellions break out. And I have to move more garrison troops out to deal with rebellions. Which keep spreading. Ten turns later, I'm now almost thirty turns away from a science victory. Which I still haven't gotten, because the other two times I tried it, I got an accidental cultural victory instead.) And I'm not really sure if I defend against the "siphon funds" attack in the city center or in the commercial hub, since the (worthless?) Civilopedia has no entry that explains it.
But the real problem with the spy subsystem is that it's a tedious game of whack-a-mole. Since you have multiple districts per city that need to be defended (almost) separately, you can easily require two or even three spies per city. With six cities, that's 12-18 targets. You will never have enough spies. So, you defend some places, and the industrial hub or spaceport that is undefended elsewhere gets hit. So you move your spies (and lose ten turns of city production just repairing stuff). It's like continuously buying a losing lottery ticket (and having to hit yourself in the head with a hammer when it doesn't pay off).
If I had an option to turn off the spy system entirely, I'd use it immediately and never look back.
