Katarina has that "Fiora Effect" (a term I first heard used by Gbay99, but very relevant)
She does nothing but mass amounts of dps, she has no real source of cc, but this makes her perfect for carrying solo queue. If other people can do just a minor amount of cc, the fed, raw dps champs, can easily come in and clean house.
The problem is that snowballing is a mid-late game feature which is meant to share your feed with your team. You become unstoppable, and make sure you win every fight, so your team can then take objectives. The problem is in the application of snowballing, where you see people become unstoppable with no counterplay. A level 6 ultimate that can be used 3 times in one minute is stupidly strong. Even if you deny the full ultimate, it hemorrhages damage making it impossible to keep her pinned for long.
I honestly wish that players would think before they complain. You are comparing a champion (lee sin) who's only good at forcing a snowball on either himself or others in the early/mid game. To a champion meant to nuke people at all stages of the game post level 6. Their purposes are completely different and they should be treated as such.
Another player mentioned a 70% ban rate in Korea. A lot of people in the NA server think Ezreal sucks, but he is one of the safest and easiest marksmen to win bot lane with, and the Koreans can prove that one. Look at Korean stats before you complain. If they are winning 60% of the time or more with just one champion, it needs to be looked at to see if it is op, or the players have found some mechanic that makes it op.