It is not know to what extent Goutetsu used the SNH.
Ryu is a mindless maniac when using it canonically.
Gouki has his mind in check, as he has mastered it, but is has still affected him, it has turned him into an empty shell almost, who thinks of nothing but fighting and killing his opponent, all he retains left of his humanity is a code of honor.
No, Satsui No Hadou Ni Mezameta Ryu HAD his mind in check.
But he just had a strong desire to kill all challengers that came before him. Gouki is not as mindless as you say he is. He trains, he still lives like a ordinary human, kinda. Capcom even drew an official artwork of him selling or buying lemons from a young boy.