See, that's what people who never used either engine usually say but the truth is: If the game is developed well, you won't be able to tell whether it uses Unreal, Unity or its own engine.
Either tool you use, you still have to program RPG functionality from scratch so it simply doesn't matter which engine you use. There are a lot of non-shooters developed with Unreal just like there are a lot of FPSes made with Unity.
The reason why Unity got so popular for isometric lower budget Kickstarter RPGs and such is because it is much easier to get started with, it's the easiest engine to learn to use, its Asset Store has a lot of products that can really speed up your development, and until recently had the best pricing plan.
Note that when all recent "big" isometric rpgs were starting development, UE4 wasn't even out yet.
I love Unity but fact is that Unreal runs better overall (Unity is hell to optimize) and isn't a bad choice for RPGs at all, especially 3D KOTOR-like ones. At the end of the day it's like arguing whether your HD reskin would look better if you made it in Photoshop or Gimp.