My main beef with the 1st was that the RNG was all-powerful.
Good games can have an RNG leaning, but also allow the player to stack the odds in their favour.
X:EU did not. You could have a flawless plan, execute well, and still be 100% screwed by the RNG. The RNG was King, and the game made sure you knew that.
I bet the game still won't let you shoot at terrain, unleashing round after round of explosive ammo in the general vicinity of where you think that alien is... That is what the originals let you do, and that's how you beat the RNG
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That and bringing 20 soldiers for every alien
X:EU, on the other hand, would say, "No sir, you can only shoot directly at that one alien we are showing you, because he's the only one not hidden by that lamp post over there. Now sir, he is a little beyond touching distance, so we are reducing your chance to hit by 5000%. No, I'm sorry, you can't shoot in his general direction. And for your information sir, a 95% chance to hit means you'll most likely miss."