So there are a few things to respond to from two different people, so I'll just tackle them in whatever order I think is most important.
First, I want to correct a false notion of what imbalance is.
I've read now numerous ways for OL's to negate, mitigate, rein in, and whatever else you want to call it, to use against the TH. Everything from using particular plot decks, OL decks, deploying certain minions and etc. You do understand the TH is not doing this alone, there are 3 other heroes the OL must also contend with. However it seems that your focus is so on him that all those things you mentioned are with the TH in mind and not anyone else. As somebody else has stated in this thread if all of this is needed for 1 hero there is an imbalance. Other classes get certain things used against them for a particular reason but no class needs that amount of attention to negate such as the TH.
That is not what imbalance is. You are arguing that because a unit of set X (one class out of the set of all classes) is better than the others in that set, that it must therefore be imbalanced. This is wrong. This is a game of 4 classes working together in order to defeat the overlord, so the overlord needs to address all 4 classes at once and the totality of what the heroes can do, and he has the totality of options at his fingertips to deal with it. In this context, the Treasure Hunter is not hard to deal with. The responses to the other heroes is not neglected, but it's not referenced because the response to other heroes is baked into the response to the Treasure Hunter (in part) and is separate. For example, to counter the Runemaster, you mostly need to make good monster selection and have good movement and positioning on the map. That's useful against the Treasure Hunter as well, but isn't an outright direct or hard counter the same way the right Overlord cards are. I also can't just tell you "control your monsters better" very effectively, or choose better monster groups, although you probably need to, if the treasure hunter employing melee weapons is killing all of your stuff and finding time to grab search tokens.
You counter specific healers by choosing how your attacks are applied, or by targeting fatigue generation (targeting fatigue generation is specifically good in general, but better or worse against certain hero compositions). For example, the bard is countered with focus fire, whereas the disciple is better dealt with by spreading the damage out. They can also be neutralized by forcing the party to split up (unless it's the phophet). The warrior is a class that you counter directly with good monster selection and movement.
The reason you want to specifically target the treasure hunter with these cards is that the heroes are making a very specific strategical statement when they choose a composition which includes the treasure hunter. They are saying that they are going to attempt to early snowball as hard as possible because the Treasure Hunter is at his highest overall power compared to the overlord rather early in Act 1, something that is not true for any other class in the game. They want to leverage that advantage early for some easy wins fast (in which case all you have to do is harass and focus on denying treasure), or to just improve efficiency of their losses (which is still a loss in the long run in the making).
You target the treasure hunter with these counters (overlord cards, plot deck, etc.) because it is the most important part of their strategic choice when he's on the field. You also, coincidentally, often target the scout with these choices when it's not the treasure hunter anyway.
So, let's step back and talk strategy for a moment: The Heroes have a few resources available to them every round: health, stamina, and actions. Their tools (skills, hero abilities and heroic feats) provide options for the expenditure of these, but the reality is that it comes down to having a finite amount of stamina and actions for actually doing things. This game, for the heroes, is primarily about action economy. You want to get as much done as possible for the fewest amount of actions. The Treasure Hunter is worse at this than most other scouts, because the class offers pretty much nothing for doing that, other than the starting accessory (dead man's compass IIRC, which gets replaced reasonably quick because you can only have 2 accessories). The Thief, for example, can search from 3 spaces away, which means they can more efficiently get there to do the search: they save 4 movement points (2 to move in, 2 to move back out). The reason the treasure hunter is better as a ranged character is that they need the extra action economy offered by being ranged: Having to move into melee to attack and back out costs a lot of movement points. Logan Lashley in particular is a spectacular treasure hunter because his hero ability is pretty much the only way to help out with that, as is his heroic feat, but even he still suffers from it.
As the overlord, your job is to screw with the efficiency of hero actions. You can do this in a variety of ways: knocking them down, making monsters survive additional attacks, wasting hero movement, conditions, etc. Proper use of overlord cards, monster movement and positioning is key to accomplish this, and the treasure hunter, even as Logan Lashley, is pretty much the worst scout at maximizing action efficiency on the map. This makes him more susceptible to specific counters than other heroes, which is another reason to target him. A well timed Imploding Rift can flat out ruin his entire day, as can a well timed mimic card (I've lost track of the amount of times the extra volucrux reaver out of nowhere got a knockdown or just plain ran away with their treasure to the corner of the map).
The Treasure Hunter isn't really any different from other scouts in the way you deal with him. You want to use your overlord cards to deny treasure anyway, it's just that the Treasure Hunter is less efficient in his actions, but more rewarding when they are accomplished, so you have to punish the less efficient part. Logan Lashley can be annoying because he's immune to immobilize and gets bonus movement, but that's not the Treasure Hunter: that's Logan Lashley, and I don't think it's broken.
I find the Wildlander to be much more difficult to compensate for over the course of a campaign than the Treasure Hunter. The same is true for the Marshall class: the two directly attack your overlord cards and force a complete rework of strategy. The Treasure Hunter just requires that you double down on something you were going to do anyway.
I write strategy articles on another website for a different game (Starcraft 2), and there are certain trends that are common. One of them is players who are facing a challenge they aren't sure how to deal with that requires a significnat adjustment from what they feel comfortable doing. This isn't an indicator of balance, but they don't know what to do about it, so they lose. A lot. And that's frustrating, I completely understand, but the problem isn't in the class, it's in the way you approach dealing with it. It's often much easier to blame the balance of something than it is to step back and completely change how you look at the game to find an answer.
I was going to reply to @Reno Shiv and others that continue to feel so strongly about the TH, and what they perceive as "balance". Now I don't have to. Very well stated Whitewing.
I have the feeling that some of us are simply going to have to agree to disagree, as I believe that no amount of cogent arguments and/or strategic tips are going to make a difference.