Kat isn't pick or ban in pro matches because 5v5 teams/pros is fundamentally different than 5v5 randoms. You've got 5 people coordinating, usually with voice chat, compared to 5 random people that might not even read the chat. Coordination absolutely crushes Katarina, and it's abundant in pro games but scarce in pubs.
She's complained about because she's rewarded for assists as much as kills. Hexakill has 1 more champ than normal for her to get a 15second cooldown reduction from, and she doesn't even need to get a kill to do it.
That's why she's cancer in hexakill: She can show up, combo+ult, maybe get CC'd, but it doesn't matter because her 5 teammates will probably get at least one kill, which gives her an assist and a basic CD reset which she can use to secure an additional kill/assist, rinse and repeat every fight.
'Just CC her' isn't enough to stop her from resetting and doing her full combo again: You have to 100-0 her within the time of that stun, or she will flash/zhonya's/run/have her team save her and then her resets start coming up off the assists. A Katarina with 10% HP is still 100% capable of brutally murdering your ass with her abilities. CC ain't enough.