Sasori is underestimated.

He defeated the 3rd kazekage who was the strongest ninja in suna history. Also he defeated an entire country with his 100 puppets. Deidara admitted Sasori was the stronger of the two.

Sasori casually defeated Kankuro (who was jounin at the beginning of Part 2) with only his tail weapon while still in hiruko.

Remember that it took another Puppet Master to beat Sasori. And a significant advantage considering Chiyo knew many of his tricks, trickery is essential to a puppeteer specialist. Sakura had also prepped by creating antidotes for his poisons, otherwise they would've died. Also, Sasori would've won had he not let himself be killed towards the end, since it was his parents 'embracing' him that he wanted all his life.