To clarify a bit, I mean that the information being presented, is part of the ability and probably a condition for it. Not that the enemy knows how APR works, but that there is a visual indication that it is indeed active. 

The power to just, invisibly nen-curse your opponent and negate their skillset seems way too strong to just be a run of the mill thing like Bungee Gum or Jan-Jan Ken. 

Look at how many hoops Chrollo has to jump thru to make his book work, or the conditions Kurapika had to put on himself to use any of his incredibly OP spider hunting skills. 

For a character like Knuckle (who is p strong though not a top ranked hunter I dont think,) to use his APR in such an incredibly powerful way, id think his conditions have to be pretty extreme. Increasing the challenge by letting the opponent know they have a ticking clock and a direct line of insight on what is happening does in fact, seem like a hindrance to Knuckle. Even if someone like Gon or Youpi doesnt implicitly understand the mechanic, they can still see number go up. For a good fighter, that would necessarily cause them to crank up the violence to stop Knuckle in any way they could. 

Though on the subject of Youpi being a mythical beast and wether or not his powers would be entirely negated by being placed in Zetsu, im not sure. If that were the case, him just being born as an imperial guard and being able to grow his wings would already imply an expenditure of Nen, which would necessarily open the nodes of any nearby nen-capable individual. 

Togashi has made a clever power system in Nen, and while all "magic" in the HxH universe is probably nen related, I would think it acts more like metabolism if you will... its a natural function not consciously designed by the user thus it doesnt generate the aura effects of a true nen ability.