Board Thread:Speculation House/@comment-10380701-20131018004029/@comment-11188061-20131018105742

Also, Weiss' spell are probably limited and not as innovative or powerful as the typical singular casters. The biggest weakness of a Jack of All Trades is that she could do everything, but she can never be the best at one thing.

A simpler explanation would be that she is the best at doing everything, but she can never be the best at anything.

I can easily see why Weiss would feel inferior.