Firstly, just because they can use Unreal doesnt mean that it isn't causing problems, even you state that they are using it juat for their graphics which inplies challenges.

It's a completely different game by a completely different team. And it doesn't appear to be released yet. So it seems very premature to be judging it based on runtime performance..

Now, personally, I would choose yo build an RTS l/Builder in Unreal over Unity over time. But that's almost entirely due to my experience with Unity the company and their antagonistic incentives towards their developers, and the ease in which it is to get source access and little to do with technical engine fit. Honestly, in my professional opinion, the design of Unity is more flexible in this regard, and I'd consider it a reasonable decision to try to use Unity from a technical engine design POV.