What specifically are you talking about? Go can do syscalls perfectly fine, you can mess in your memory perfectly fine (even if you have to resort to the unsafe package), so what exactly is it where you think Go lacks flexibility compared to C? Because so far, your posting sounds rather hand-wavy.
One problem is that it's very hard to call go libraries from other languages (it requires a runtime). C libraries can be called from almost any language.
There are many go libraries I would love to use, but can't because of this. I expect many others are in a similar boat.
Go simply can't interface everything the way C does. Go doesn't give you the same level of flexibility either.
tldr: Go's great, but it's just not as powerful as C (unlike Rust or some others, which can do anything C does).