Hiro has too little control...perhaps Peter, but he's too hesitant. Thus far, the Haitian and Eden have dispalyed the greatest control over their abilities, and since the Haitian can negate her, I'd have to say it's him. We also don't know if he can indeed negate abilities, so it's hard to say his abilities are limited to negation of min abilities. As for why he didn't stop Nathan from flying away...maybe it's because he needed to have some understanding of WHAT exactly the person's abilities are, so it's not on auto, like peter's mimic abilites or sylar's siphon.