User blog comment:Omega natsu2/Chapter 394: Juvia vs. Keith, Predictions./@comment-3285005-20140720052301/@comment-7418318-20140720081634

I wouldn't even be upset even if he was the weakest of the Demon Gates. Just that he's weaker than the other eight doesn't mean much. If he's just a little bit behind the others, that still leaves him very dangerous. Just look at Jura. During the Nirvana arc he was the "weakest" of the Wizard Saints and yet he was still quite the monster and beat Brain with barely any effort at all, despite being wounded from Gimini's backstabbing.

Alternatively this "weakness" could just be Silver having the same prejudices that people often have about Celestial Spirit Mages. They think "Okay, this guy hides behind his revived corpses, so he must be too weak to fight by himself".

Or Keith could be fighting economically, putting in just the minimum required effort to fulfill the task. If he never shows his true power that could also account for him being misjudged by his peers.