Hmm, that's a good point about the Zetsu, but I think Knov was just mentally weak more than anything, and the Aura basically shattered him. Gon and Killua both felt the malice of Pitou's aura in close proximity, and didn't turn out like Knov. We were told that emotions and Nen were very closely linked, so I think Knov was just psychologically not ready for an aura of that level. Especially a malicious aura like Pouf's. I think he could have had his Ren up in-full, and he still would have been crushed if he felt that Aura. Maybe if he had more time to mentally prepare himself, he would have fared better?