I'd like to start by emphasizing for any readers of the thread that no other 4X comes close to the features and potential of Galciv 3 and SD's constant support. I've played Galciv 2, 3, every Fallen Enchantress game and Sorceror King so though I'm not hardcore, I've enjoyed SD's games immensely and enough to provide feedback about why I can't bring myself to sit though another game.
Even though the AI, features and polish are amazing, there are issues that emerge after you've played dozens of hours like me:
1. Tech tree brevity: I have resorted to setting the tech rate to slowest and even this doesn't help for too long. Galciv 3 is a game that can last many hours in each session. However the tech tree doesn't seem to last through the game. After you've won just a single war or two, you start progressing at breakneck speeds through the tree, often hitting the end of several lines most important and just mopping up random tech as your game just devolves into a battle of the gods and ship spamming. It really is disheartening in a game to know that making strategic choices about tech doesn't matter much when you're gonna hit the end of multiple lines soon anyways, taking out much of the strategy behind it.
2. Alliances: Speaking about the main weakness of Galciv 3 and indeed every 4X game, the endgame is weak. The AI does not seem to make many alliances with each other and if you start snowballing, none of the other AI's band together and ally to stop you as the greatest threat, or any other threatening AI. This makes the endgame simply you eating one isolated AI after another, getting more and more overwhelming as time goes by.
3. Lack of Endgame Fun: I certainly have fun in the early to midgame, but I often think what's the point when I know it's gonna end soon. The endgame can drag out the longest. I have taken to modifying the Dread Lord appearance to be much more frequent. But I believe that Galciv 3 could benefit greatly from more endgame threats. Stellaris' endgame crises and powerful fallen empires are a great example and Shogun 2 Total War's realm divided mechanic where the AI's band together against you when you get too powerful would make for great additions.
Ultimately these 3 things are the only issues I have. The pro's of Galciv 3 still outweigh any cons and make this the only 4x for me, I just wish SD would break its habit of dull endgames which has plagued not only Galciv 2 and 3, but also all of the Fallen Enchantress games as well. These are issues that plague all other 4X games but I believe that SD has the ability and creativity to overcome them.