Who brought up Overeem? I agree that Nelson would have his hands full in that one, simply because Overeem is Hyooge and a very devastating striker. But, I think that Nelson would have just as many problems with guys like Cain, Lesnar or Carwin because of their size/strength/wrestling ability. Cain and Carwin also hit like they've got bricks in their gloves.
Overeem vs JDS would be in Alistair's favor IMO, because JDS's best arsenal is his striking, and Overeem is a K-1 champ. So, in theory anyhow JDS would be at a disadvantage from a stylistic match-up in that one.
Overeem vs Cain though might be a different story as Cain's best arsenal is his wrestling. While Reem has steamrolled his way to victory over a few big (read: heavy) guys in Strikeforce lately and won the K-1 Grand Prix in 2010, he hasn't really faced any big, strong, top notch wrestlers. This may give Cain the advantage from a stylistic standpoint in that fight.