Isn't Law & Smoker vs Vergo a perfect example how haki nullifies fruit powers? But it also comes down to how strong the haki is. If the haki isn't strong enough it can't nullify. Both smoker and Law was dominated completely. And only Law's last attack that was strong enough to slice the entire base and mountain managed to finally hurt Vergo. And that is how i always imagined haki to work. I have a hard time to believe Garp and Shanks and others like them that has duked it out with the strongest of them wouldn't be able to nullify most of the fruit damage and effects.