Overeem is Cain's kryptonite, been saying it for a while.
Cain can't strike with him at range or in the pocket, Reem's boxing/muay thai is too crisp and accurate. He can't clinch with him and dirty box, Reem has probably the best clinch at heavyweight, very strong and those KNEES. His best route would be to try to score takedowns and GnP from top position but Reem has a great guard with good subs and sweeps and decent wrestling himself, and if Reem were to get top position his GnP is absolutely brutal. On top of these stylistic comparisons, Reem has shown off great poise and execution of his gameplans lately whereas his own emotions and recklessness have been his downfall in the past.
If the fight were to go down, I'd bet on Overeem by TKO within 3 rounds.