I said 'core' of a last-gen engine - it's going to have limits, heavily modified or not.
How technically impressive you find it I guess depends on your perspective, but I find the scaling options and subsequent performance of whatever engine Shadow of Mordor uses to be much more commendable. I do think it is impressive that Rocksteady got AK working so well on consoles at least, given the low performance GPUs but they have ample VRAM so optimising for less was never an issue with those platforms. It is a nice looking game but I don't see it as technically impressive on PC.
For my part I'm really in awe. You can tell immediately it's UE3 but Rockstready have managed to make so much with it it's incredible. SoW does not have anywhere near as much going on, so it's no surprise it runs better overall.
Really as much as the game had unpredictable performance before the patch, post-patch it's nearly perfect on my system. Once again I put emphasis on "my", because I can't guarantee to anybody else a good experience even on beefy specs.