You gotta really learn to be flexible with these kind of gripes one may have with an anime.
Yes, it makes him absurdly powerful, but it literally can only be used on a few specific people, tying into his lust for revenge.
What would you rather it be? Him training a la DBZ and going on a rampage? That wouldn't be nearly as interesting.
If this made you angry, hoo boy would you have raged at a certain moment in the Chimera Ant arc.
Except it's not just "thinking"? Dude had to train his body to be 100% psychologically connected to chains by locking himself in a room with them, licking them, painting them, listening to them, and overall developing an insatiable fetish for them so that when they got taken away he started hallucinating them, which allowed his Nen to form them.
He didn't just snap his fingers and go "YAY! I can make chains!"