Whilst some of the issues here are potentially the fault of DOTS and HDRP not being production ready (in combination at least) there's plenty of blame to go around.
Elsewhere other people are making gorgeous and performant games in Unity.
And remember, despite Epic's efforts, UE is still a first/third person engine at it's core. I wouldn't nr surprised if the challenges using Unreal would be at least as great if they had chosen that.
All the issues are around rendering and HDRP - which by the sound of things is still not properly integrated with DOTS.
And their data based ecss is still behind DOTS for simulation last I checked
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.