According to this thread, as of version 4.6, the maximum supported multiplayer map size out of the box is 20km x 20km. Is this the limitation you were referring to? That wouldn't accommodate an MMORPG or Minecraft, but otherwise seems quite big.
Of course, what matters more is what you do in that 20km, but it's also a limit coupled tightly with distant mesh performance and float accuracy issues.
Regarding the editor, unreal is pretty amazing.
...and that's a big IF. The editor ran like absolute rubbish originally on macs, to the point where it was actually unusable.
They've done a lot of work on that, but it's still miles away from the unity editor.
I think it's really hard to argue the unreal editor is better; it's a lot slower, but it does have some nice tools.
You also can't create extensions as easily (if at all?) as you can for unity.
As you say, they're two different things, and the unreal editor is actually not bad... but if you were to compare them, the unity one would certainly rank more highly.
Now, the multiplayer map size limitation seems odd nowadays.
I believe there's also some hard coded limits around the number of simultaneous players (16?), though it seems fairly easy to work around. Again, not sure about performance.
Regarding the editor, unreal is pretty amazing. It really shines in the tools department. Check out the landscape editor, materials editor, and animation tools on youtube. Pretty fantastic stuff.
The game engine itself is better than unity in almost every way with a few big exceptions:
1) multiplayer map size. unity can pretty easily support large online worlds 2) support for mobile/tablets. unity is the dominant platform on those devices. 3) community support. unity has a much larger community, better documentation, and a much more populous online store (though unreal is catching up)