It has been tough for me to admit to myself that Civilization 6 is, in its essentials, a fundamentally broken game. I have been playing the Civilization games since Civ II/ Alpha Centauri. I have also, in between releases, played many of the turn-based strategies that clearly owe a lot to Civilization, including the Galactic Civilizations series, Age of Wonders, Total War, etc., all since the early days (I realize that I am betraying both advanced age and a misspent life here). Civilization always felt like the original and for that reason special.
Ironically it has taken a lot of work to break this game so completely. The root of the problem lies in a slow progression of intended improvements to the game which have both totally outstripped the abilities of the AI and have erected impediments in the ability of human players to do things that used to be straightforward. Many of the core problems arise from the decision, in Civ 5, to de-stack units in an effort to make combat feel more tactical, and then impose this tactical combat framework on the strategic map. That decision created loads of obvious and basic problems that many others have already commented on in the context of reviewing Civ 5: e.g., tactical battles that stretch over hundreds of years, a confused crowding of the campaign map with units, etc. Because there was a lot to like about Civ 5, I had hoped that Civ 6 would find a way to improve some of these issues. Unfortunately, they seem to have gotten worse.
The first and most obvious problems with Civ 6 relate the complete inability of the AI to play the game. Computer players in Civ 5 were dumb, but in Civ 6 now they are beyond dumb- they simply cannot function in a way that is rational at the most basic levels. With respect to discrete tactical battles that involve more than a couple of units, the AI is utterly helpless. It neither exploits advantages or guards against weaknesses- not even to the limited extent the AI could do these things in Civ 5. The AI is even worse when it comes to managing armies at a strategic level. The AI is utterly unable to mount an effective invasion force even between two neighboring cities on the same continent, let alone on a broader scale. Adding water tiles to the mix completely flummoxes the AI, as units without escort repeated kill themselves by hurling themselves at your naval units and well-fortified coastal cities, without support or the slightest possible chance of success. Forget the significant overseas naval invasions that the AI would sometimes surprise you with in Civ 4: that is completely beyond the capabilities of this game. I have played Civ 6 quite a bit already have yet to have an interesting tactical engagement with a computer player (fighting of hordes of uncoordinated horsemen is not interesting). On top of all that, for some reason, the AI in Civ 6 seems much worse than ever before at advancing itself technically, and seems to prefer spawning hordes of older units to mindlessly throw at you. I am not saying that I am some genius and that I am so much smarter than the AI. Rather, I am saying that a lobster is smarter than the AI. In sum, the dynamics of turn-based tactical combat on the hex-based strategic map are just too much for the AI to handle.
But the problem goes beyond just the AI and the Computer players' inability to play. In essence, I can understand why the AI has problems, because I too find it exceedingly difficult to assemble and move even a modest-sized mixed-unit force from one place to another. Because units can no longer be organized into stacks, each step in the process of mustering an army, moving it, transporting it, supporting it, and attacking with it is a horrid logistical mess. And because of that, each step takes multiple turns, so if you try to do it in an organized way, many years will pass (hundreds of years if you are early on in the game), units will obsolete, etc., all of which is just dumb. And again, Civ 6 exacerbates rather than alleviates these problems by adding layers of additional technologies, structures, and districts needed to recruit and organize a mixed-unit army. For example, I used to enjoy putting together a solid force of land units supported by a navy, and move them over to assault coastal cities. That was fun in Civ 4, a pain but doable in Civ 5, and torture in Civ 6. At the most basic level, Civ 5's experiment with attempting to overlay mixed-unit-tactical combat on the strategic map has failed on multiple levels, and Civ 6 has amplified that failure. The AI can't handle it, and although it can be fun in limited circumstances, for the most part it doesn't work and is more of a pain than a benefit.
I'm not sure what to recommend. An improvement in the AI would be a big help, but given the complexity of this game, I am not sure that is possible for any AI to handle it. Another option would be to allow a partial or total stacking of units, which could not only help the AI, but solve a lot of other messes created by the hex system, discussed above. Regardless, something needs to be done, as the game really does not work as it stands.