Leonardo da Vinci overrated as an engineer/scientist?

One often hears harping about Leonardo’s uncanny mechanical designs, including flying machines. But, was a working, practical model ever built from any one of his designs? Are his designs sound from a modern engineering point of view, only it was not possible to construct the parts with the materials and technology of the time? For some examples, see these.

Or did the social, religious, and political context make it risky to attempt to build any of these?