I'm going to try to keep this to the point.

The X series wasn't always that complex. The first two X games were more accessible some say (I haven't played any of them though), but with the introduction of X2, a space-sim with some empire-building elements turned into a full-fledged empire-building game with space-sim elements.

X3:Reunion, X3:TC and X3:AP have all in turn refined and added complexity but also depth, making the game harder and harder to pick up for new players, but also more fun and rewarding for veterans of the series.

I'm quite used to it now, though even now after years of playing there still are some aspects of the game left for me to explore. From what I remember when I first started to play X2, I can definitely see a rookie's point about the steep learning curve. It's more of a ravine climb, but if you ask me it's well worth the effort.

Rebirth on the other hand broke this chain of complexity and in-game depth and returned the series to simpler times, with an increased focus on the player and his actions. This is why you'll see people complaining about it in the first place - some feel that this oversimplification betrays the direction the series has taken so far.

Personally, I see their point. I don't see myself playing Rebirth, and I'm much happier to stick to X3:AP for now. But if I had never played any of the X games, I guess Rebirth would be a good place to start. From what I've read it's not a bad game at all in itself, it's just not what us, X series veterans, are used to.

Still, if you want an open-world sandbox where you can do whatever you like, earn money, build factories and fleets, explore, board ships, grow yourself from a single starting ship to a massive empire that can overpower the all-mighty Xenon - I think X3:AP is what you're looking for. Expect it to be hard - and it should be - you'll get as much out of it as you put into it.