Recently played and lost to a Poppy. The second she hits level 6, those 2 man dives for the guaranteed kill start happening. Her kit really isn't bad at all, I don't understand the hate.
It's not hate so much as that her laning pre-6 is absolutely BRUTAL. If her opponent has any ranged harass at all, she may as well just afk at tower, and even if they don't she will get zoned HARD if she falls behind even a teensy bit. She has no real escapes except a speed boost so she's super susceptible to ganks, and doesn't farm that well under tower.
THAT SAID, she is an absolute monstrosity if she can make it to Sheen/defense, and late game she is one of the hardest of hard carries, gibbing carries or basically anyone in 2-3 shots. It's just an issue of it being really, really hard to get there without lots of jungle help.
Honestly I can't think of a champion who is truly a liability to the team who doesn't have a place in a proper team comp played by a skilled player.
That being said champions like heimer/poppy/sion are already "being reworked" because... well just look at them.
heimer and poppy just don't fit within the current styles of play, sion simply has an identity crisis. Heimer either works on a early push/poke comp... or goes mid and outplays some summoner to get really fed and matter. Poppy doesn't have enough tools to survive until the summoner has farmed enough gold to 1v5 the team.
Champions like trundle and sejuani are being brought up to a different level of play.
Really Rengar/Eve are probably the "worst" picks at the moment and they don't even function improperly (but no one likes playing a champion that requires to start snowballing at 2 minutes).
Warwick/Syndra both play well and need more community love.
Warwicks problem is there are more and more top laners (common picks) that just bully the heck out of him than there was in season 2.
Syndra's problem is skill cap. Same with quinn.
Vik is another champion that will probably receive some love soon
But yeah with the proper skill as a summoner any champion can take anyone from placements to diamond