All the low-level crypto is written in assembly. And not only for speed, but for ensuring properties like constant-time execution, etc. It's the same for OpenSSL, and the same for commercial crypto libraries. Go is not at all different here.

The difference is that the higher-level crypto is written in Go, not in C; Go is memory safe, much more strongly-typed in general, and with run-time bounds checking which eliminate buffer overflows.

The bugs are almost never in the low-level algorithms, they are in the higher-level components.