The core problem of CAD importing in my experience is that CAD content is not at all designed for or suitable for direct game engine importing. It requires a ton of cleaning to be optimized.
It's one thing to bring in some terrain and an appropriate overlay of imagery, but another thing to try and import actual CAD content.
As an example, I had the CAD files for a large industrial printing press at work, and thought about importing them into Valve's latest engine for VR. But the CAD files were TOO detailed. I mean literally the threads on each screw were modeled, and even the interior of the lock on a cabinet that was part of the machine. Every single component on circuit boards was modeled. It brought Hammer and the game engine to its knees with all the detail.
Another issue with CAD is that not only can things be incredibly overly complex, but the model can include details that the end user simply has no need to visualize, and hence no need for the engine to try and render. Like the interior of that generic key hole latch on the cabinet. You don't need to model the interior of a pipe let alone some complex interior valve unless there is a reason to actually see that detail. 99% of the time a cylinder will do just fine.
Then there is kinematics and animation. Want doors to open and close? Objects that are moveable by humans to be physics objects? You have to do all that yourself. Why would you do that? Well because that's part of an immersive environment that creates a truly engaging and informative experience for the design you are working with.
In the end it was easier to draw on my past experience as a level designer. I selectively imported some details that would be difficult to reproduce, then hand built other components out of simple shapes and apply textures that showed the necessary detail without actually modeling it in detail. And if I did need detail, I could remodel a specific part at a higher level of detail and take advantage of built in level of detail management for models to keep the engine happy.
Summary: nice idea, been talked about for ages, not really that easy if you actually want to build good interactive content in a game engine.