The issue with Sona...

She is a generalïstic support that is supposed to fit any comp since she can do a lot of things but isnt extremely good at any of them. The concept is fine in and of itself unless you consider there are is another that also fit into the same niche: Nami.

Nami's W alone combines almost sona's entire kit in one ability and is also safer to use. Whereas sona will have to go in AA range to land her poke (and thus opens herself up for counter-engage), then use her W and possibly E to get away from any counter poke/engage since her entire poke is frontloaded into her Q powerchord and she has nothing left for the next few seconds (pre-6). Also: since her poke is AA based she also draws lovely minion agro and given her squishy base stats that's always fun to have as well.

Meanwhile, Nami runs up to the enemy at a much more comfortabel range, throws out a splash of water that hits the enemy, bounces back to heal Nami and give her a speedboost due to her passive to boot. literally Sona's entire poke routine that requires all of her abilities combined into ebb and flow and her passive. It also doesnt trigger minion's to chase you across the lane like a pack of vicious bloodhounds.

On top of that Nami has Hard CC pre level 6, her passive outperforms sona's E, she has a slow she can give to others (sona has to use a powerchord if she wants one which comes at the expense of dmg or dmg reduction) and her primary lane harras skills also provides her with sustain should you find the need to change your playstyle, Sona has to choose between better poke or sustain and improving one comes at the cost of being worse at the other.

Tidal wave is for all intents and purposes a slower, but also wider and longer ranged crecendo that also slows. only thing Nami doesn't have is a mini exhaust in the form of powerchord diminuendo, but again, Sona has to sacrifice damage or a slow for it. one could also argue that her aqua prison essentially provides her with a 50% dmg reduction for 3 seconds (rather than 20% for 3) since you can't deal dmg while suspended (with of course the added benefit of being able to land it on more than one person).

Does this mean Nami outperforms Sona in every aspect? yes, when it comes to lane she does... But Sona is a teamplayer and her strength grows the more people she has to support so. while Nami is far superior at supporting one ore two people, Sona shines in a team fight situation. except, the ability to save, support and peel for one carry is often much more desirable than bringing a bit of support to everyone, the only difference would be when facing a comp with a lot of Aoe dmg. you would expect that to be where Sona thrives right? Again, not exactly... to gain the full support of Sona people have to bunch up together since the aura's are rather small, yet that is exactly what you don't want to do vs a team with load's of aoe. the only situation in which you would expect Sona to truly shine she can't because it would risk certain doom. The thing that is supposed to be her biggest strength is also her Achilles heel.