Not with Monocypher. That project has one implementation defined behaviour (right shifts of negative integers), and it's one where all platforms all behave exactly the same (they propagate the sign bit). In over 5 years, I haven't got a single report of a platform behaving differently (which would result in public key crypto not working at all).

However I do get spurious warnings, such as mixing arithmetic and bitwise operations even in cases where that's intended. Pleasing every compiler is not trivial.