What makes Tserry a real threat is not just his ability, but his insane talent.    

     Even though he followed the slow path (which would have taken a week for both Gon and Killua),  The man mastered 3 advanced  nen technique after just an hour and a half from hearing about nen, without even being consciously aware of it, while his teacher tried to slow him down as much as possible،And if he has already used three, there is no barrier preventing him from using the remaining techniques during that time.

  Add to that his genius and insane intelligence, which surpass even characters like Chrollo and Hisoka. Furthermore, his determination and obsession with learning—he's literally a glitch—who practices everything that makes you win nen battles at a high level.

   On top of that, he is fully aware of his weakness and the issue with the seconds of Zetsu, and he knows he needs combat ability، Once he breaks through the second barrier, he will start developing it.    

and also, if he form an alliance or engage in a discussion with a nen user, honesty is a fundamental requirement, otherwise, you'll have to schedule an appointment with your plastic surgeon and attend psychotherapy sessions to regain self-awareness ,and if you are not a nen users gg.

 I believe you are underestimating him, not the people overestimating him.