Wildlander has the best ability in the game, hands down, and for 1 xp. No other scout class can force the overlord to discard cards. When properly used, it can completely eliminate the overlord's ability to forfeit encounter 1 of a 2 part encounter to save up extra cards for an overwhelming advantage on part 2. If the overlord isn't rushing for a win in part 1, you just take your time and keep forcing him to discard cards.
Wildlander can consistently put out good damage more reliably than other classes, and has more good abilities. Yes, the permanent 2 piercing ability for the treasure hunter is very strong (yes, I think the treasure hunter is the second best scout class, but not at all overpowered), but the Wildlander gets a wider array of useful abilities. Further, the overlord's job is to counter the player's choices. If a player picks up treasure hunter, I usually just grab Raythen's plot deck and laugh my ass off. Even if I don't, you can usually stop the treasure hunter from getting to pick up a lot of treasure by targeting him, blocking him off, or simply forcing the heroes to rush and skip treasure or lose.
NImble is a fantastic starting ability as it drastically improves the survivability and mobility of the wildlander. Properly used it can get you into position quite a bit quicker, or force the overlord to waste actions trying to reach you. Accurate reliably improves your damage output each round, and also for 1 xp. Bow Mastery guarantees you a surge, First Strike can get you a free attack, Fleet of Foot is a huge movement boost, Running Shot is spectacular. The only Wildlander ability that isn't outrageously good is Black Arrow, and that's quite decent still for the range boost and guaranteed minimum damage.
In comparison, the Treasure Hunter has fewer abilities that are particularly good. Dungeoneer is very situational, but potentially good. Gold Rush is okay, too situational, and it gets worse when you actually hand those potions you pick up to your teammates, limiting the heroes options. The problem with these two abilities is that a lot of the time, the treasure isn't where the action is. Survey is okay, improves your odds of getting something good and the bonus stamina is nice. Guard the spoils sounds cool, but does nothing when you draw the best card (treasure chest) or the secret room. Further, it gets worse when you hand the search cards to your teammates, often preventing them from being used by the person who would most benefit. Lure of Fortune is mediocre, especially for 2xp. Sleight of hand is spectacular, and along with delver are the two reasons to play this class.
As for the thief, he's not particularly good at anything. He's worse at dealing damage AND getting treasure than the treasure hunter. His abilities are more situational than any other scout, and frankly the conditions he applies aren't all that useful. Sure, he can stun or immobilize, but to do that he's gotta be in melee, and scouts and melee don't mix all that well. Thief is just plain worse than the treasure hunter is basically every way.