Sona's damage is fine. I have no qualms with her poke, nor her late game damage (it can be pretty absurd to take 50-70% of a squishy champ's health with a Q + Stacatto Power Chord combo- with a LOT of AP- and I'm fine with that). But frankly, her W is extremely inefficient until late game (assuming it's rank 5 with over 300+ AP at the very least) and her E is just... well frankly there's nothing to it. Late game, rank 5 (crescendo rank 3 also) with 300+ AP and MS items it's useful. Still doesn't give you incentive to rank up until you have no other option to do so.