I tend to agree. But not in an absolute manner. In the real world, Kitchen and machine shop tools demonstrate the effectiveness of 3D interfaces. It’s because we can give and receive input in rich ways and use whole body thinking techniques. In a computer screen / flat land, 3D interfaces are way more cumbersome usually b/c of keyboard & mouse input

"3D outputs paired with 2D inputs are cumbersome" seems like a good takeaway, contrast 2.5?D outputs and inputs in environments like dynamicland i.e. "using slightly protruding 3D buttons sticking out of the UI surface in places where you would otherwise use color changes or faux-3D effects like bevels or drop shadows." as per the post

Also useful to consider everyday 3D encumbrances: clutter, the junk drawer, etc.

Consider these 3D interfaces: (This is really 2 demos on one, the other starts at 6:00)