Kuroro cut open Silva's skin and Silva remarked "that's a good knife", implying that the sharpness of the knife is mostly responsible for the wound. If you stab a Reinforcement user in the head, it's still going to get through and you wouldn't need to make a very deep wound to kill someone when you stab them on the head.
Now of course nobody is going to be crazy enough to try to block a knife or a fireball wtih their head, but that's what you could end up doing if you can't handle In. Here's a very simple case. As a Materialize user, I use no weapons at all. I materialize an invisible sword and use it to cut through your neck. It doens't have to be very deep wound, because if you sever one of the main arteries near there the other guy is dead. They say the stuff you can materialize has a strength comparable to top quality real items, so if a knife can cut Silva (clearly one of the most powerful users), a materialized sword should be able to cut Ubogin too. You won't be able to cut very deep, but if you cut in the right places, you wouldn't need to.
Of course Ubogin's is described as someone having top notch physical skills too, but you can't dodge what you cannot see. There are just far too many ways you can lose to In in a variety of combos if you don't know how to handle it. Again, given how powerful In is, it'd be basically impossible for anyone to actually survive more than a couple fights without figuring how to counter it every single time because the time you missed it is most likely the time you'll end up dead. Therefore you should never be tricked into not using aura vision because it's simply too risky.
---------- Post added at 02:06 PM ---------- Previous post was at 01:51 PM ----------
Actualy it's not even just limited to Reinforcement users. Say you got 2 Materialize users and both materialize a sword. It'd be reasonable to assume their sword can kill each other (both would have lowish physical defense due to distance from Reinforcement). So what's stopping the first guy just pretending he has no weapons and then use In on his sword and kill the other guy? After all if you see someone with no weapon you'll probably assume he's Reinforcement or Emission, and then you'd die to the sword + In combo.
It's not so much as what Kurapika did, but rather in the world of HXH it'd be unfathomable to survive without knowing how to handle In. You'd just randomly have your head chopped off or your face mauled by a fireball, and the first time you failed to counter In is likely to be your last time. Certainly we see failure to counter In leads to devastating consequences, and honestly neither Chain Jail nor Hisoka's bubblegum is an ability designed to kill you. As Shalunark said, it'd be way easier to materialize chains that paralyze you compared to Chain Jail, and that + In would definitely kill anyone too if they didn't see it coming.
---------- Post added at 02:20 PM ---------- Previous post was at 02:06 PM ----------
Here's another one. Gon's scissor projects aura in the form of a sword and it's basically like a long claw attached to his hands. Given he totally sucks at Transformation, it can be inferred that projecting aura in the form of a sword/claw attached to your hand is pretty easy. This can be hidden by In, so you can just keep an open palm, try to hit someone, and if they don't see the claw/sword they have a chance to die from that thinking they only need to stay away the length of your hand. So even a Reinforcement user can kill you easily with In if you don't see it coming.
There's literally an endless number of ways you can die horribly to In if you don't know how to handle it, so it's pretty much inconceiveable that anyone from the Spiders do not know how to handle it, because if they don't they'd have already died. I don't mean this means that Ubogin is a newbie. I think he's just the last person to die to In before Togashi also realized there are an endless number of ways you can die to In and it'd be pretty lame if every fight goes "Oops I got tricked by In".