So the Maven of the Strings has proven to be a bit of a touchy subject for some people, including Riot themselves. She's gone through a LOT of changes over the many years since her release (she was one of the first few champions released after I started playing in Season 1) and while she's remained a popular pick in casual play, has seen an almost 100% drop-off in professional competitive play. According to lolking's stat tracking, she actually has one of the highest win-rates in the game (at about 54.33% as I type this), despite being considered almost completely inviable by pro teams.
Why is this?
Well personally I attribute this to Sona's various reworks complete inverting her original design of "incredible lane bully who drops off into an ult-bot late game" to "squishy lane liability who becomes an unstoppable support goddess late game". In short, Sona has become the Master Yi of supports.
Consider her role, for starters. The _entire point_ of a support in lane is to "support" their ADC lane partner to get them through the lane phase, either by letting them dominate their opponent (as bully or kill-lane supports like Zyra or Leona do) or simply nursemaid a weak early-game ADC (like Kog'Maw) through their dangerous period until they can look after themselves (like Janna does). Sona _used_ to be one of the game's strongest bullies due to her Q>passive combo simply chunking people out with embarrassing ease, but a succession of nerfs have severely weakened her damage compared to the likes of, say, Brand support, while her weaknesses (an almost 100% lack of pre-6 CC and her incredible squishiness and lack of mobility) have become even more prominent. The lack of CC in particular (she has a very weak, conditional slow on her E Power Chord) is CRIPPLING during the laning phase, rendering Sona almost completely incapable of peeling for her ADC, and the sustain from her W is initially so weak as to be almost irrelevant. Pre-6, Sona is just flat-out terrible and can only survive by managing to trade damage against squishy, sustain-lacking opponents without taking any hits in return, ironically negating her supposed advantage of being "easy to play". Only the improved mana recovery from Ancient Coin (and the power of Ardent Censer) even keeps her playable.
However, the biggest change Riot ever made to Sona was adding the inbuilt hard CD reduction to her ult. At level 6 it's a mere 10%, almost meaningless, but at level 11 it jumps to a flat 25%, and by level 16 she's getting 40% off her cooldowns. A level 18 Sona has _incredibly_ short CDs on all her basic abilities even BEFORE you factor in CDR from items, runes and masteries, meaning she can spam her abilities almost non-stop. Even the relatively weak heal and shielding off her W becomes incredibly powerful when it's coming every 3 seconds or so, and her E can let her entire team fly around the map like they're on roller skates. Plus she gets quite respectable damage out of her Q, and Crescendo is, as ever, a reliable 1.5 second AoE stun. Additionally, Sona needs quite a few items (for a support) to negate her serious mana cost issues and let her reach her full potential, most notably Ardent Censer.
With this in mind, Sona's position becomes clear: late-game champions are perfectly viable in solo queue because a lot of games run much longer than they should have and teams aren't as capable of exploiting her obvious weaknesses, so a "support who takes the entire game to become good" is something you can actually get away with, like Master Yi jungle (another pick you'll pretty much never see in professional play). Sona's power doesn't peak until level _16,_ when she gets the 3rd rank of her ult and the full 40% CDR on her base abilities. A late-game hypersupport is something that simply has no place in pro play, where Sona and her ADC would both be utterly destroyed in laning and the team playing her would have their nexus down before 25 minutes, never letting her accomplish anything, but is something quite powerful in slow solo queue games.
Sona is an anomaly, almost a refugee from a totally different game, one where it's the _ADC's_ job to carry their _support_ through the lane phase. And in solo queue, this makes her quite a strong champion, just like Yi and Tryndamere and Vayne and Nasus and all the other "I take 30 minutes to get any good and then I kill everything" champions. Sona has always been perceived as a "noob champion" solely because all her abilities except Crescendo find their targets automatically, but it may even go further than that to the very core of her role in a game (Yi, for example, is also considered a "noob champion" and a pubstomper). I wonder if this was actually Riot's intention when they last reworked her? Regardless, she does provide something unique in the lineup of support champions in LoL.
Plus she still has the best tits in the game, of course.