There are three problems I see with khopeshes. I'm not saying they aren't great weapons (they are) but they are not as easy a fit on a rogue as they are on other classes.
Problem 1: Khopesh users don't tend to use picks against held mobs like rapier users are. The x3 crits are "good enough" to not require making GS picks. So khopesh users who don't switch to picks are actually doing quite a bit less damage against held mobs than rapier users...but claim they do way more. Khopesh > Rapier for held mobs but Picks > Khopeshes.
Problem 2: The extra feat that khopeshes require often force a TR'd rogue to miss out on the fantastic (and fun!) active past life feat, Sneak of Shadows.
Problem 3: The need for the extra feat often prompts rogues to splash a level of fighter instead of staying pure. That is a HUGE reduction in DPS for a small amount of gain.
But I wouldn't write off the khopesh-wielding Half Orc Assassin based on this. It's a devastatingly powerful build. There are lots of ways to build a rogue and HO assassin is a good one.