Janna still has her uses. Her ult is still the ultimate teamfight disengage, her passive is still very powerful, her Q is still one of the best support skills in the game. When I say Janna's bad, that's shorthand for "Janna is generally not as good as most other supports except in the specific instance of facing an AoE team comp, at which she is awesome but that situation does not crop up very often".
And similarly, while Sona is overall very strong, there are situations where you wouldn't want to take her. Versus Kat and Nunu I'd want Alistar or Taric. Versus winmore tops like Irelia or Jax, or for general teamfight control, I'd want Lulu. Versus hypercarries like Kog and Vayne I'd want Blitz or Leona to really curbstomp them early, or Lulu to neutralise them late. The problem with Sona is this: if these champs are 9 or 10/10 at these specific roles and 5/10 at everything else, Sona is at least a 7 or 8/10 at everything.
It's the curse of the jack of all trades in this game - the more things you do, the harder you are to balance. Either you're too weak overall to really stand out, or you're so versatile that there's no reason you wouldn't get used.