> ...just wondered why the gargantuan effort with a DIY engine.
Given that Jonathan Blow is in charge of the project, it's almost certainly because existing engines were inadequate for the task.
From what I can tell, Rust is the most ambitious Unity project out there. The Witness is substantially more technically ambitious than Rust. Additionally -from what I read on the Rust devblogs-, the Rust guys spend a large amount of time working around Unity's limitations, missing features, and parts of Unity that are just buggy. (Granted, the buggy stuff generally eventually gets fixed, but until it gets fixed (if it ever does), you have to work around it.)
Actually, I think Frontiers (http://explore-frontiers.com/) might be even more ambitious Unity-using game, but the developer working on it is in over his head. Still, what he's done so far is amazing already.
Given that Jonathan Blow is in charge of the project, it's almost certainly because existing engines were inadequate for the task.
From what I can tell, Rust is the most ambitious Unity project out there. The Witness is substantially more technically ambitious than Rust. Additionally -from what I read on the Rust devblogs-, the Rust guys spend a large amount of time working around Unity's limitations, missing features, and parts of Unity that are just buggy. (Granted, the buggy stuff generally eventually gets fixed, but until it gets fixed (if it ever does), you have to work around it.)