Disclaimer: I am not an authoritative expert on shanty town, but I haven't really seen someone who is. Constructive criticism is welcome and encouraged.
Shanty Town is the oft-maligned cousin of Village. At 3$, it shares a price point with the vanilla Village, which alone tells us something. It is not a village+ (like Worker's Village, Mining Village, Walled Village and Mining Village at 4$) or a Village- (Native Village at 2$). It is a card that plays entirely different from the plain old Village, and shares only the characteristic of giving two actions.
A good way to think of Shanty Town in many (but not all) games is as a restricted lab.
When is Shanty Town a good buy?Shanty Town does not have a single thematic board type that makes it powerful. Rather, a piecemeal combination of a bunch of different cases make it a card that I would argue is more often a good buy than a vanilla village
1)
Terminal-Less decksIn games where there are no terminal actions worth getting (notable cards to cause this are Familiar, Caravan and Tournament) early on, Shanty Town functions identically to a laboratory if you buy one, and each additional one functions as a slightly riskier lab (as your two shanty towns can combine). A lab for 3$ is nothing to scoff at, and while shanty-town is not deck making in these games it provides a significant boost.
Keep in mind that you can often still keep 1-2 terminals in your deck if your deck is large enough, the collision chance is small enough that buying Shanty Town will still benefit you.
2)
Draw to X EnginesThe draw to X engines that Shanty Town works best with are Watchtower, Library and Minion (which functions as a draw to X). It's an iffy prospect with Jack of All Trades that I would not recommend.
Shanty Town is superior to the vanilla village in Draw-To-X engines, though it is still inferior to Fishing Village (what isn't?). This is because there are three possibilities with shanty town in a draw-to-x-engine:
A) It is drawn with with a Draw-To-X-enabler card (Library, Watchtower, Minion)
B) It is drawn with no other terminal
C) It is drawn with another terminal that is not draw-to-x
Possibility A is a relatively likely option, and in this case, Shanty Town is nearly identical to a vanilla village. (Its ever so slightly worse with Watchtower and basically equivalent with Library and Minion, due to library's filtering and minions discard mechanic)
Possibility B is the second most likely option, and in this case Shanty Town is +2Cards +2Actions,
far superior to village.
Possibility C is relatively unlikely in most draw-to-x decks, and is either significantly worse (no need for +actions, terminal not draw) or slightly worse (need for +actions or terminal draw card).
Conclusion: While Shanty Town is not amazingly better than village in draw-to-x, it is enough of an improvement to merit significant consideration in these decks.
3)
As a counter to attacksShanty Town is much more likely to activate fully when you are playing it against attacks, primarily discard attacks. Shanty Town reacts slightly different against different ones, however.
Militia Shanty Town is only so-so against Militia. Often you would rather keep another terminal (say... a militia
) as opposed to trying to recover your deck with Shanty Town.
Ghost Ship Shanty Town is an excellent counter to Ghost Ship. This is because instead of discarding your terminal actions, you can topdeck them and then draw them with your activated shanty town. This actually is almost a
complete counter to ghost ship in many cases, because if they hadn't play ghost ship your Shanty Town would not have activated (both situations leave you with 4 cards and 2 actions).
Margrave Shanty Town is nice with margrave because not only does it provide a bit of a counter, it also facilitates the terminal draw aspect of margrave if you have it in your own deck.
Followers If your opponent has followers, then it is a tournament game. In which case you probably wanted a shanty town anyways to go with your tournaments.
Torturer Shanty Town does not really counter torturer very well because you're unlikely to have actions outside of +actions and torturer itself. However, since torturers are on the board you probably want a good amount of Shanty Towns anyways...
Goons See Torturer, really.
4)
In games flush with actionsFishing Village is your biggest culprit here, though certainly it's not the only case. If you have enough +Actions available to you on a given turn, you can simply play your terminals before your shanty town and then use your shanty town as a lab. In practice it plays similar to case 1, terminal-less decks, just with less of a guarantee.
5)
When you need the +Actions regardless. OK, so shanty town is worse than village in cases. Suck it up, sometimes you will still desperately need +actions in your engine. This tends to be true in torturer games, megaturn games involving terminal draw, and many others. Even when it's worse than a normal village, it's still a village variant and sometimes that's what you need. A good litmus test in these cases is that if Shanty Town has no support, if you would buy native villages for your engine, you probably should still buy Shanty Town.
6)
With Massive, bloated decksIf you have 15 coppers and 5 curses in your deck, Shanty Town is far more likely to give you +2 cards. 'nough said.
When is Shanty-Town a bad buy?1) With medium to heavy trashing! Trashing your starting coppers/estates makes it much more likely to collide and not activate. In these cases you need to evaluate if there's better alternatives and only get shanty town if you desperately need the +actions.
2) Most Terminal Draw goes pretty terribly with Shanty Town, especially envoy, embassy, smithy, and council room. Attacks are less bad because you more frequently want to build an engine around them, and because shanty town counters some of them. Vault is meh-probably take silver instead, but it makes little difference either way.
3) Big Money games involving a few terminal actions, draw or not (Jack of All Trades, Merchant Ship, Masquerade, etc.)
In conclusion: Shanty town is situational, and only sometimes can really be viewed as a village, and should not be maligned as a worse alternative to village.
Works with:Draw to X, especially minion.
Torturer
Ghost Ship
Caravan, Tournament, Familiar, Other cantrips and nonterminals.
Bloated games caused by cursers
Fishing Village (though they conflict at 3$ and FV is usually better)
Powerful Engines
Doesn't Work With:BM-X, where X is terminal
Games with moderate to heavy trashing that aren't powerful engines needing +action
where the next hand has been drawn and the provinces are empty, but the game hasn't ended. 
.