According to this chart which I stole from Guardian.gg, Sunsingers have the second lowest KDR, which is equal to Nighstalkers. While Bladedancers have the highest KDR, gunslinger coming in second.
If it was really overpowered, people would be doing amazing with it and the KDR would be much higher. What this means is that the abilities are not strong enough to override skill, and therefore, not overpowered.
Edit: the purpose of the chart is not to call anything OP. All the stats are within 0.10, which is not that big of a difference.
The actual KDR value of sunsinger is 1.01 while bladedancer is 1.11.
All the classes are essentially equal.
NOTHING needs a nerf.
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Class k/d is a bad way to balance things off. Nightstalker, bladedancer, and gunslinger have high k/d's because the higher skill players stick with them and don't jump around like the lower skilled, players who judge a class purely off the super.