As matter-of-fact, she's actually one of the weakest characters in the entire game. At the very least, she's undeniably the weakest support currently in the game. Her W and E are both practically nonexistent spells, she has one of the weakest ults of the whole class and her damage output is lower than any support that isn't named 'Braum.' Not to mention she dies in literally 2 hits or less, yet has no pre-6 CC and sits in the lowest MS-tier despite this fact.
She's a useless character that has to desperately rely on artificial numbers from non-support items like Spellblade and Echo to do literally ANYTHING at all. Without these items, her true winrate falls below 40%, which is significantly lower than any other character in the game.
It's not an exaggeration to say that she's completely unplayable and this was a direct cause of Gutting 4.13. Having a Sona on your own team is a near guaranteed loss, I've even dodged final promotion matches on multiple occasions because it's more logical to accept the loss and just move on rather than suffer playing with a Sona on your team.