Im sorry to tell you that that couldn't possibly be more wrong. Actually 6p Naruto's Standard has one of the worst (if not actually the worst) Reset of the game. Compared to what you are thinking, naruto doesn't reset his mystery just by comboing any ninja, he needs to combo ALL ninjas he hits with his standard, which instantly makes dodge ninjas, and block ninjas (like kimimarou or han) an instant non-reset for him, unless you bait them before getting hit (which is possible when his mystery activates every round).

His standard combo rate is comparable to that of KCM naruto, with that only exception, but he also gets a lesser combo buff by using his mystery, so it isn't strange to see him NOT comboing even with that. This is the reason almost everyone that uses 6p naruto uses him with a Fan ninja tool, because it increases combo rate, which is fundamental for him to "do mystery every round", he actually needs a lot of it, and if you see him comboing in teams with han or kimimarou that often, theres three posibilities:

1.They have really low combo stat in those nins (Kimimarou for example, his block isn't almost 100% guaranted unlike Han's) and so Naruto can just combo them by overwheelming stat difference.

2. Naruto isn't hitting that ninja with his standard, remember there is usually 4 ninjas in a team, its not unlikely naruto will hit the other 3 ninjas without dodge/block and reset his mystery.

3.They are using bee or any team that makes naruto do standard 2 times, and i do need to explain this one? also this is the most common team used for 6p Naruto currently, if you are fighting one of these, they will most likely be able to reset his mystery, even in the first round.

If you have doubts about it just try him in arena and see yourself how terrible his combo rate and reset is compared to other similar ninjas. Outside arena the stacking (which is when you over stack with power your position 1 ninja, so it has the highest stats from your whole team) will make naruto much more effective and he will reset much better. Actually, almost every mystery reset ninja relies a lot in combo stat to be viable, with the few exception of Yagura edo tensei, or swimsuit temari, whose mystery resets with their chasers.

Naruto relies a lot in his mystery to be useful, or "OP". There's no need to keep saying there's already counters for him and that he isn't as good as many are painting him, he relies more on his team and your BP than you and others might think.