Issue is she's too much everywhere. Each spell cast doesn't seem to feel relevant enough regardless of actual efficiency, her base stats and her spells without AP feel too low for the support role and a myriad other problems.
TBH I love the current Sona Gameplay to death. But I love her as a midlaner where she can cheese enough depending on matchups to get fed and dominate the early/mid game through Sheen/Lich's Bane procs, and later become a bit of a monster using her auras.
My finished full build gives around 600-700 AP depending on variant items, that means her Q projectiles will deal around 500 each, plus around 200 on an AA per ally tagged. PowerChord will add another 400 (without Q 40% buff) Total efficiency may seem sky-high for a single cast but at full build any mage can fairly easily outstrip that damage in some way or another, not to mention how it's going to be spread around.
She'll heal around 250 to self plus to one target, and shield around 300 to all who get tagged, BUT since that shield merely lasts 1.5 seconds, the large majority of the time it'll go down before being used. The cumulative heal is fun when you're just going around healing peeps while sieging for instance, but in an actual fight the spell itself feels limited in use.
Her E is the most controversial. The only time when it's really worth anything is on an AP Sona. With scaling, the spell goes from 20% of base speed to 60% at full build.
Those numbers are kind-of high, but this is endgame gear on a straight AP build. Without those scalings, which really don't feel all that high everything considered, a Support Sona will feel her casts are close to useless.
The issue is, buff anything and she risks becoming overpowered really easily because she scales with nearby teammates for efficiency.