so what is the counterplay to the unknown?

The innate counterplay is obvious: Look at the Unknown as much as possible when you are Weakened, dispel Hallucinations whenever you see one if at all possible. If the add-on that applies Weakened on finishing a generator is in play (a strong add-on that is of merely green rarity), let go of gens before they pop if you are with someone else so that only one of you will get Weakened. If someone is already Weakened, they should be the ones to finish the gen, of course. General counterplay is abusing distance whenever you can and abusing close proximity if you can't. The latter means getting close to the Unknown behind pallets or windows, because your average Unknown player will have trouble making the UVX connect up close.

That said, Unknown is definitely oppressive in chase, and the teleports afford it good potential outside of chase as well. Unknown is much stronger than the average forum populace will make it out to be, it's another case of a somewhat more demanding killer with high potential that an average player will find too difficult to do well with and consider to be bad and the game imbalanced after not doing well in a handful of matches. If you watch good players play him, you'll see them demolish (such as Knightlight, who is on a 50-or-so winstreak with Unknown as of now if I'm not mistaken, usually winning at 3+ gens left standing - some examples of as much here:

There's also this example of a 1v1 event:

The average and median chase here lasted 50 seconds, only a handful of people going above 1 minute in over 40 attempts. And keep in mind, most of all of the players participating here are already much better in chase than your average pub survivor, as they are not only experienced players but specifically experienced in 1v1 pure chase gameplay, and follow that scene enough to watch and compete. Also notable that they have the entire map and its resources, two health states and two chase perks at their disposal, as well as no previously-spawned Hallucinations around the map, all luxuries a survivor in many scenarios in a normal match would not have. And yet still they on average die in under a minute. It can also be argued that there is more to learn playing as Unknown than there is to in playing against it, the pragmatic skill ceiling is definitely higher on the killer side because they can make crazy shots happen; it stands to be expected that with time, players will disproportionally get better at playing Unknown than they will playing against it (particularly of course in a pub environment, where the survivors maybe face Unknown once in a blue moon while the killer player may even main it). So the fact that Unknown is relatively new only makes this even more impressive.

I think what's particularly problematic in chases is the fact that Unknown can hold and cancel the UVX at will. The zoning and faking potential this affords them makes it nigh-impossible to reliably play around, as dodging may just simply end up netting you an axe in the face. I had various suggestions in my PTB feedback, but BHVR unfortunately deletes that. Simple suggestions were to reduce the time you have to look at Unknown to lose Weakened or to have other means of losing it to do with the Hallucinations, as well as just changes on cooldowns, movement speeds while and after using the ability and so on, while a cooler suggestion was to make it so Unknown can't hold the UVX but that once the player taps M2, they have to commit to the shot, making it much more reliable to play around. Of course there should be some compensatory buffs as well, simple ones being the ability to break pallets with the UVX, a more ambitious idea that of being able to detonate the UVX at-will at any point of its flight.

But I doubt BHVR will do much of anything, and to be fair, I don't think Unknown is in a terrible spot. Fairly oppressive and at times just actually impossible to play around, but not more than solidly A-tier either, with good add-ons yet none that are really busted (not to my knowledge at this time anyway). I do think some slight rebalancing would be in order, but there are much more pressing issues in the game.

There isn't much to add here that hasn't already been said, but it is good to reiterate that removing the hallucinations is a must against him.