I mean, it feels YF2 a.k.a. FurryVNE is written from the ground up, but from what I've seen lately from Unreal, it feels it flows better than Unity in various aspects, also that UE can be highly customizable for a wide variety of devices, and its ultimate version UE5, feels like a jewel for VR and amazing stuff.
Again, I'm not pro on any of these engines, but I just feel it has higher amount of characteristics found in UE5 than in Unity.
Even so, the developers have years of experience working with Unity. Switching to Unreal would require them to learn a whole new set of tools and also a different programming language. FurryVNE has to do some pretty advanced stuff to get itself working, and the developers having an intimate understanding of Unity is one of the things that make their custom components (like what they're doing with soft body physics) possible.
Just consider how it feels when you switch to a different piece of software in a particular domain (Photoshop to GIMP, Premiere Pro to DaVinci Resolve, Microsoft Office to LibreOffice, Windows to macOS/Linux, etc.). It basically does the same thing in the long run, but getting to your previous level of mastery takes time. The same is true here. The same concepts often apply, but you'll suddenly need to figure out how to get from point A to point B again, and even doing basic tasks can take quite a bit of discovery.
UE5 isn't really relevant here as it didn't exist when YL2's development started. Moving between major engine releases in mid-development is almost never a good idea. Even moving between different UE4 releases could cause major headaches, so if we lived in an alternate timeline where YL2 was started on UE4 it would almost certainly stay there.
It's also worth noting that the specific features you brought up (multi-platform and good VR support) are features of both engines. Unity can publish to any platform from desktop to mobile to your web browser, and it's very prolific in the VR space (popular examples: H3VR, Beat Saber, Pistol Whip, Boneworks, and even most of Valve's "The Lab" VR demo). Despite it being the poster child for shitty asset flips, Unity is a solid, feature-rich game engine.
tl;dr: the important part is that the engine has the features the developers need and that they can work efficiently with it. In YL2's case, Unity ticks both boxes.