This seems pretty simple to me.
LeB is mechanically undertuned. Not just a little bit undertuned, but majorly undertuned. Or in other words, she is under-powered (as opposed to over powered). The reason she is undertuned is because she ate some VERY heavy-handed nerfs due to her kit being one of the most non-interactive kits in the game. Playing against her is frustrating at best. At worst she's nightmarishly oppressive in lane, far worse than Karma is.
Or, in other words, she's 'broken'.
Broken in the sense that she breaks the design values that League has been trying to set up over the past few years and thus has a kit that can't be buffed, only reworked.
She's a pretty common pick still over in high level Dominion play. The loss of Silence made it possible to actually retort against her damage if you have enough targeted CC, but given that Riot is really keen on crushing targeted CC every chance they get, it leaves options limited.
Pairing her up with AP Shaco is just the worst thing.
The worst.