While duration's basic constructor is constexpr (as is it's default) allowing for:

constexpr Seconds foo(20);

It's implicit converting construct is not. Disallowing:

constexpr Milliseconds foo = Seconds(20);

This reduces the readability of Duration's (compared to standard durations) when constexpr semantics are required.

I think that in the past we avoided this because it requires a constexpr duration_cast, which is painful to write in C++11. Extended constexpr in C++14 should simplify things though, so I'd like to revisit as part of the C++14 codebase epic.