A common problem people run into with CAD models is importing a STEP file and modeling directly off of geometry in it. They later find out that some face they used as a reference was read by the CAD package as 89.99999994 degrees to another, and discover it's thrown the geometry of everything else in their model subtly off when things aren't lining up the way they should.
And that's with a file that has solid body representation! It's an entire new level of nightmare when you throw meshes into the mix.
The heart of any real CAD package is a geometry kernel[1]. There are really only a handful of them out there; Parasolid is used by a ton of 'big name' packages, for example. This is what takes a series of descriptions of geometry and turns it into clear, repeatable geometry. The power of this isn't just where geometry and dimensions are known. It's when the geometry and dimensions are critical to the function of whatever's being modeled. It's the very core of what these things do. Mesh modeling is fantastic for a lot of things, but it's a very different approach to creating geometry and just isn't a great fit for things like mechanical engineering.