I'm not too sure on how these tiers work, but if you're talking about spamming and playing unfair to the opponent (and not giving him a chance) then you're right, Sasori is powerful and probably one of the best. But the fact is, every time you use a jutsu or even dash as a puppet user, it leaves a big opening and you're vulnerable. And also, Sasori's awakening hits very hard but is very slow and full of openings..
He is S tier; if you want the official tier list from a competetive UNS2 community (it was from Arashi, the original #1 UNS2 competetive website, but it has basically died and most of the players are here now)
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[Also Kankuro is A tier mainly due to his beast awakening before you ask, he's basically B/C tier, all factors have been involved

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I think Cin explains how the tier lists are drawn up; this has been made and disputed for around a year, so there's been plenty of time for discussion and relative change, but the main reason why Sasori is S tier is due to the facts stated, but I'll go tl;dr Sasori has 2 GUARDS, powerful cShuriken, the best tilt in the game [if used correctly, if you do the 3 part tilt, you fail], an average u1 and an average u2, but one that is easy to land off SB, and his awakening is still very powerful if used correctly, I mean his combos are incredibly slow, but his regular and cShuriken are fireballs, and his u1 becomes a fire barrage, which is pretty damaging.
I think though it is mainly down to the fact that Sasori has 2 guards and an amazing tilt - competetent Sasori users are literally unbeatable, because they will block every incoming attack, and KNJ every grab, and then trap you in an infinite tilt combo, if you aren't good at KNJ'ing, you lose, I haven't even played a competent Sasori user yet; but here's some examples of one of the best Sasori's I know [he unfortunately doesn't play anymore, but his status is almost legendary xD (well especially as a troll hee hee) ], this was recorded from a live stream, where he went about matches only using Sasori's tilt - the first part got deleted due to Copyright BS, but here's the second part
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The people he plays are pretty bad and he doesn't use Sasori to the fullest extent, but the effects of the tilt can be drawn up
Honestly though, I'm not here to fight a "pro-Sasori" battle xD. I'm a serious Peinis enthiusiast and I would state that Peinis is S tier and the best in the game, but that would be incorrect unfortunately, I hope they bring him back and buff him in UNS3, I love him as a character, but his slow cooldowns leave something to be desired, I'm just going with what I know is the best overall source of authority for judging characters in this game generally, alot of time and dedication has gone into the tier list, and many arguments; I have witnessed them! xD