Compared to the other Ardent champs Sona has a longer CD heal, and burns more mana than any except Taric (Who can apply it more frequently than any other ardent user and in two different AOE's) She is also the squishiest of all of them, making her very vulnerable to ganks and doesn't have any hard CC before level 6, making her quite weak at and against turret dives.
She also trades consistent damage for burst, making her weak in the constant trading most lanes partake in, meaning that after she bursts someone she is very weak for the next 5-10 seconds.
Her main strength is her ability to reduce the enemy carry's damage, but in lane that means giving up most of her own damage, and late game it is unlikely she can do it since her range isn't great.
Another big thing is that like Taric she makes her team want to group up for the ardent boosts, but she is so squishy that the AOE abilities that are thrown out in retaliation to grouping up will just kill her in comparison to someone like Taric who is tanky enough to survive that.
Her heal also can't be targeted, meaning it will often heal the tank instead of the ADC because tanks are the ones getting taken down low first, then sit around at low HP for a while. This means that if the enemy team chunks your tank and forces them to back off, then focuses your ADC, you have almost no ability to heal them since by the time they are low enough for the projectile to target them they are already dead because her heal projectile isn't very fast.
So Sona lacks a lot of the tools to help priority targets stay alive due to the conditionals on her abilities once you get to the teamfighting stage, and her early game lacks the hard CC and tools to make up for that.
What Sona is good at is rotation games, due to her teamwide movespeed boost. However that isn't very big right now and if you did want to grab a support for that Karma is a much better choice with a stronger laning phase and teamwide ardent censors in a much larger AOE.
played more in this 
Support Meta? You can spam her heal which also grants a small shield to all players in range. It basically applies 
are just so much stronger and she is when they are banned
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