"Kate had no chance against Ari with that scalpel, he was just goading her to try it. And of course she was going to make a lot of newbie mistakes, Kate was on protection detail with the Secret Service, she wasn't a trained criminal investigator so of course there is going to be a learning curve.
As for Unsealed and the Meat Puzzle, she had to watch more than one person in both episodes; while being distracted both times allowing for the villian to move in. Even Gibbs said it was his fault he should have assigned an additional agent for each principle target.
It was far worse situation in Jet Lag, when both DiNozzo and Ziva blew their protection detail watching one person on a plane.
Kate Todd was portrayed as a smart tough modern woman and special agent that was a far more believable character and not some character coming out from a comic book. Yes she could be too trusting at times, but I blame that on the writers, it seemed to me that they dumbed her down in Season 2."