The X-Series has one huge problem: It became huge.
The simulation (God Engine, I think) worked great in X2, with the only "screw you, game"-moments coming from Khaak-Clusters jumping into your trading routes.
In X3:TC, the introduction of the Xenon M7 (which is able to beat ANY other ship in a 1on1 fight, simulated out of sector) broke the game.
I took on 2 of those M7s at once in my Vidar, without breaking a sweat. But if playerskill makes that much of a difference, it's just silly.
X2: great game, with a good balance of "places to go", "ships to fly" and "stuff to do".
X3: larger game, with too many "places to go", "ships to fly" and "stuff to do". It gets tedious to fight enemies yourself, because even the Vidar (M6) is strong enough to take on M2s, if piloted by a player. Sending in the AI however would probably require several M7s.
TL;DR: Egosoft broke the AI, it is necessary to do everything by yourself and it gets plain tedious.
I still think that X2 was great.