No, it's still type safe (http://en.wikipedia.org/wiki/Type_safety), memory safe, and free of undefined behavior. There's is no way to use a type contained in an interface as the incorrect type (without the unsafe package, but that wouldn't need an interface anyway), or to inadvertently access or modify arbitrary memory.
I also wouldn't consider Go type assertions to be in the same class as reflection. A type assertion is the way you extract and check the underlying type from an interface, and is quite common even when using more complex interfaces. There's also no way to make use of the underlying type from an interface without first asserting it as that type.
I also wouldn't consider Go type assertions to be in the same class as reflection. A type assertion is the way you extract and check the underlying type from an interface, and is quite common even when using more complex interfaces. There's also no way to make use of the underlying type from an interface without first asserting it as that type.