Because he is, just like Zamasu, a prodigy at fighting and ki usage. Plus the fact that Zamasu tends to take the blows for him if it's possible. Only way to beat him is just to be that much more powerful. Though I suppose that's true for all DB villains.
But seriously, Goku is not a noob at controlling God Ki. The only way to get that is to have perfect control of your Ki. Goku has shown that he has an amazing control by also using Kaio-ken with it.