How about the excuse that it is absolutely the buggiest game I have ever played (and I have played some buggy games in my time)?
Look, the game was fun, I really enjoyed playing through it, and I'm glad I bought it. But it suffers from a few things:
1. Awful, literally game-breaking bugs. I didn't really do anything weird or abnormal; I broke my game and had to reload a previous save 3 times before I could finish the game from bugs that didn't let me proceed. What if I was playing Iron Man? That's a game over.
2. "Questionable" (to put it nicely) balance. Well, really, there isn't balance. I guess that's fine, it's not really designed to need balance, but it still would have been nice to not have a few particular strategies and skills be flat-out better than anything else. Good luck finding someone who declines to user a Double Tap Sniper with Squad Sight, for instance, if they know what that actually does.
3. Unnecessary mini-games involving shooting down UFOs. Why? Adds nothing. Guess it doesn't detract either, except for wasting my time.
Again: I liked the game, I'm glad I played it, and I think it is good. But I can easily see why someone wouldn't have picked it for this list.