In Uncharted 1, the enemies had more mobility on Hard and Crushing and were constantly trying to circle you, so you were forced to dispatch them more quickly and to jump out of cover more often instead of being able to stay in the same place throughout the whole shootout. I sort of missed this in UC2, where they stayed a lot more in cover, but this was offset by the much superior arena design (which includes better enemy/weapon placement), shorter waves (IIRC in UC1 you could have 4 waves in the same location!) and bigger variety given by stealth and pulldowns.
In Uncharted 3, the encounters feel like a step back to the first game: the enemies are more mobile (to favor melee), but instead of trying to circle you they seem to be coming frontally right at you, ignoring your bullets. This was exacerbated by the more open ended arena design, certainly meant to give the player even more freedom, and the decision to spawn later waves before the last enemy is dispatched, to make them feel a bit more seamless (even though this had the opposite effect on me, as they were often people spawning from locations they weren't possibly hiding in throughout the fight). Both made the encounters more confusionary, as the designers had no way to predict where the player would be located or which enemies would still be around, thus giving the impression of them just throwing everything and the kitchen sink at the player instead of giving him some carefully crafted challenges to overcome (like in UC2).