Even in theoretically terrible lane match ups for sona, she has so many tools with almost no downside that it's still entirely possible to win those lanes without really trying. Sure, hard engage risks blowing her up, but if she abuses early game before any real threats appear, she should make it near impossible for any aggression from the enemy lane until they b. An aoe heal shield and a damage dealing, aa resetting, aoe aa steroid with the added ability to deal even more damage after she uses a few low cd spammable spells makes any opposing lane with no inate sustain lose the lane if she abuses her tools.
And it's damn hard to blow up a sona at level 2, even with blitz graves or leona graves or annie graves because she's basically just going to hide behind her caster minions until your adc walks up to farm. Blitz aint gonna be able to grab that without hitting level 3 for overdrive and powerfist pressure, annie could do it, but if the sona gets out, then she basically had a favorable trade because she's gonna just top herself up, and the same issue with leona.
And if you channelled all your damage on to sona and failed, chances are the enemy adc put down a lot of damage on you, so now you should be in a really bad position against any healbot. Your lane is probably at half health, while enemy adc at close to full or 3/4 while sona at 1/4 to 1/3. It will very quickly become your lane at 3/4 while enemy adc at full and sona at 3/4.
tl;dr: sona has a weakness, sure, but it's damn hard to take advantage of it because it's almost exclusively at early game where you can take advantage of it, and support items like sightstone make that window much smaller. plus she has a ridiculous power spike at levels 1 and 2, which make abusing that weakness very risky.