Katarina's the queen of resets. If she gets the opportunity to come into a teamfight late, it's really bad for you.
Basically, you really want somebody with hard CC to deal with Katarina. When Katarina jumps in, she's actually using her escape to do so, so if you interrupt her ult, she's completely vulnerable. This is why Kat falls off at higher elo, when people are a little more reliable in terms of actually bringing CC and saving it for the right target. In Silver it's still not uncommon to see people pick a comp with literally no hard CC against Kat.