I've played Evochron Legends, and nabbed Mercenary last year, during one of the Steam sales. It's fun for a while, but despite the size of the universe, there's not much uniqueness to it. Everything feels pretty generic, and not really alive. And contrary to ScriptWolf, I believe the X series offers you more stuff to do, and a better chance to build some sort of empire. The universe is just more fleshed out across the board, but the problem with X3 has always been it's crappy single-core engine, which struggles to give 60 FPS whenever there's anything more complex than a single station on screen.
But back to Mercenary, yeah, there's not much to do. Once you realize Fulcrum Torpedoes can give you a big profit margin, you'll be making millions and soon have your ship maxed out. That might have a bit of an exploity feel to it, but the alternative is doing boring repetitive missions for paltry monetary and reputation gain, which is quite MMO-ish. Or mining or some saner trading, but the result is the same. There's a lot of planets out there, but they're all bland and uninteresting, and there's technically rare anomalies and hidden places you can discover, but you can't hope to do so without some sort of guide unless you enjoy spending many hours on end traversing black, empty space, hoping to stumble upon something. But that's very unlikely, given space is big. And by big I mean HUGE.