I don't think she's bad per se, just that she could use something to push her in a direction that actually matches her design theme. Syndra as a character is all about surpassing limits and achieving untold power but in game she plays as a lane bully that falls off massively as the game goes on. Her passive compliments her design: unlock new power on her abilities when you hit max rank. Her playstyle of extreme single target burst and massive 100-0 pressure very early in the game does not.
She had the same problem that old Veigar had, namely that her kit forces her into 2 separate directions: one around scaling and the other around bulling enemies. Old Veigar had his Q stacking mechanic too but most good Veigars just used their Q to harass their lane opponent rather than farm, reach 1 shot potential in around 15-20 minutes, and then fall off from that point on since you can't 1 shot people any harder than you already are. New Veigar fits the scaling hyper carry mold much better and his kit is more coherent as a result, and I'd like to see similar changes to Syndra that makes her les of a lane bully and more of a ticking time bomb.