If you're able to put LB behind in lane, she's screwed for the entire game. She loses the ability to farm without taking enemy harass or blowing all her mana, and if she falls behind her damage sucks and she loses every trade because she has one of the lowest HP pools and crappy base stats. If she's behind and does no damage, her ganks fall short.
For years she was much stronger than she is now, able to delete squishies with a single QR combo, and she had a silence, and nobody complained about her being OP, and if anyone even suggested it, they were immediately downvoted and laughed off the forum.
A lot of champions just simply have much better teamfight presences and if they can get through the first 15 minutes without feeding kills, or if they even manage to kill LB once, she's put in a bad spot for the rest of the match.
Liss is one champion that counters LB really hard - Liss can root her every time LB jumps in, can dash out of her chain before it procs, and Liss can even completely stop a LB combo by ulting her mid-dash, negating all damage, and almost guaranteeing a kill on LB every time.
There are many others that beat her, but Liss is one of the best counters, and she's not in-meta, so she's never banned.