I don't really get Urchin either. It seems the attack is innocuous at best. Yeah it's a cantrip so it doesn't hurt you to have it, but it's at best slightly better than Spy. Mercenary is decent, but since the trash is required, it's not that easy to be able to use it. All-in-all the whole thing doesn't impress me. I'm sure there are great things to do with it, but I'm not sure about how frequently you can actually pull it off. Seems like you mostly need some way of drawing a lot of cards as well as gaining cards. That way you get the Urchins kind of for free, and then you can get stuff for the Mercenary to trash. That seems to make it too situational for me. Though I would not be surprized if I were completely wrong about it...
The things I'm sure are misplaced are Lookout and Tunnel. Lookout fear is unjustified. Just don't play the Lookout. It's not the only trasher that becomes useless late game. And it's a really good one early game. Compare it to Masquerade. It cycles the same amount and trashes the same amount, and it's non-terminal. The drawback of course is that it doesn't help your current turn if you don't have a way of drawing the card you left on top, but then it still helps your next turn. Now clearly it's not actually as good as Masquerade because helping your current turn is a big deal, and you can still use the Masq as basically +2 cards late game. But the fact that it's at least kind of close in the early game for engines should be worth something...
Tunnel I probably underrate it a bit, but 12 is way too high, imo. For the most part, Tunnel is just a big Estate. And Estates are bad, mostly. It can't provide enough VPs to really matter most of the time, and it's not an action card, so it requires significant support to really do anything. And even then, all it does is gain Gold, which is not really all that great a lot of the time, since adding the Gold is only offsetting the pollution of your deck with Tunnels. Sure there are some really powerful combos with cards that draw and then discard (Young Witch, Vault, Storeroom, Embassy, Warehouse). But outside of that, it's very situational.