Old-timer here, playing since Civ II. It's of course way too early to compare Civ VI with the rest of the series in any comprehensive way. I want to make two points. First, in my opinion, Civ VI is in relatively good shape at launch. Second, the problems with combat AI in both Civ V and Civ VI are troubling and may never be adequately solved. Overall, despite its flaws, I think Civ VI has great potential.
First, as others have correctly noted, Civ VI is in much, much better shape than its immediate predecessor was at launch. The Civ V launch was almost a total disaster; unfortunately, that seems to have been forgotten by too many fans. Nearly every facet of Civ V at launch was broken, both in terms of design and execution. The game's lead designer suddenly left Firaxis shortly after launch. It took years--years!--for Civ V to get into decent shape. I'm not sure that Vanilla Civ V ever got into good shape. Only after two expansions, at the very end of its run, well after Brave New World shipped and was polished, can we say that Civ V became a very good game. Civ VI is in much better shape at launch, despite some significant issues (diplomacy seems completely broken, the balance in science and production seems way off, there are several very annoying bugs in the UI, and as I'll discuss below, the AI is not good enough).
Second, the combat AI problems common to Civ V and Civ VI are troubling because it may be the case that combat with the melee/ranged unit separation and one unit per tile (OUPT) system is too difficult a challenge for AI. In my experience, the combat AI in Civ IV: Beyond the Sword was excellent. It presented a good challenge. There was a back and forth in combat with the AI. Of course, Civ IV had a much simpler combat system: there were no ranged units. Units could stack up, infinitely, on the same tile. Pathing and movement was simpler as well. The Civ IV combat AI, in short, could play.
In Civ V, the introduction of ranged units that actually attack from a distance, the addition of complex movement/pathing rules, and the restriction of OUPT each present tremendous challenges for the AI. In all of my hundreds of hours in Civ V, the combat AI rarely, if ever, presented a real challenge. There were always obvious mistakes in its tactics--putting ranged units in front, allowing unit after unit to die in the same chokepoint, retreating when it should be attacking, attacking from the wrong direction, etc. etc. Many of these issues were simply never fixed.
The most troubling thing to me about Civ VI so far is that its combat AI seems to have similar limitations as Civ V's. In my first few encounters, I've seen the combat AI make a lot of obvious mistakes. Perhaps it is slightly improved over Civ V. I think the new movement limitation--you can't "cheat" by climbing a hill at the end of your movement--helps the AI pathing. So does, in theory, the ability to pair civilian units to an escort. That should cut down on AI mistakes. My early impression is that the AI has sometimes chosen targets well--it attacks my weaker cities. I've seen some effort at coordination of the AI's ranged and melee units in ways that Civ V's AI rarely pulled off. It's also retreating damaged units appropriately, sometimes. This is all encouraging, but overall, my first impression is that the combat AI will just never have the ability to play as well as it did in Civ IV (and earlier games).
Bottom line: Civ VI is in better shape than its predecessor at launch, and despite concerning flaws in its combat AI, there is a lot of potential. It's too early to come to any definitive conclusions.
Civ 3 was my fav for a long time.
