As both a designer and primarily a developer I can tell you there hasn't been a design I couldn't implement exactly as per spec over several years.
The issue has been more a case of it would be better not to and the design being incomplete (being most frequently static mocks) it hasn't fully captured the interaction, all scenarios or device widths. Designers need to really understand the medium they are working with and meet devs halfway.
Good design is function over form and working within constraints. All too often in the industry the designs are created as though they are for print. This is in large part also due to the fact designers tooling itself was designed for print and we are just now beginning to have better tools for designing interfaces but there is a long long way to go.
The issue has been more a case of it would be better not to and the design being incomplete (being most frequently static mocks) it hasn't fully captured the interaction, all scenarios or device widths. Designers need to really understand the medium they are working with and meet devs halfway.
Good design is function over form and working within constraints. All too often in the industry the designs are created as though they are for print. This is in large part also due to the fact designers tooling itself was designed for print and we are just now beginning to have better tools for designing interfaces but there is a long long way to go.