Well, to tell the truth, the underlying crypto functions are also the ones harder to get wrong, using test vectors. The hardest part is probably trying to make them constant-time, but AGL is also the author and maintainer of Go crypto library, and he did extensive research on constant functions, and contributed code to OpenSSL as well. He even hacked valgrind to check for constant-time at runtime, which blows my mind (https://www.imperialviolet.org/2010/04/01/ctgrind.html).

So I think the general advise not to rewrite a TLS library doesn't fully apply to the Go team like it would apply to us.