Coming from a place of the majority of my experience being Unity experience and 6 months or so of Unreal Engine. There's a nice feeling of "It just works", especially from the Unity background. The UI was the only place that gave me some trouble for a bit. Especially since when you search for documentation a lot of the links just don't seem to work.
This was a top result on google for searching for something networking related:
https://sbox.game/dev/api/Sandbox.Network.GameNetworkSystem/OnConnected
Yet it goes to a dead link for me, had someone else confirm it was dead as well. There's tons of these so I had to rely on discord searches far more than anything else. There's even links from the old documentation that link to the new https://sbox.game/dev/api and those are dead as well so it can be hard to find what you want.
Other than that it was really smooth, since networking mostly works out of the box things feel much better. Unity has bounced around a ton between networking systems, with my first major game I made I used Unity's new (at the time) UNet which was immediately discontinued after my game was released and took months of work to move to a new system. So it'll be interesting to see how long term support for these things goes.
When external builds are fully working I think just as a standalone game engine it will definitely be able to hold it's own.