I have equal shares in Unity and UE4 and know both engines quite well.
Ark is certainly not bad looking, but compared to The Forest is looks not that good, especially when you look at gameplay on youtube posted from users, not the developers.
About the multi-threading and optimization mentioned a few posts earlier: I am not sure, but if it comes to real next-gen graphics and to how much things you can squeeze in on screen, then it might be a drawback that UE4 is younger than Unity.
UE4 will catch up with multi-threading and optimization, for sure, but then Unity will also move forward. I wonder if there will a hardware requirements gap for ever - that is UE4 needs better hardware than Unity for about the same things.
The Forest (Unity 5)
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ARK user gameplay
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