(Is it even a hot take if it's been over 4 months since the expansion came out? Who knows.)
In general, "Gain X Horses" can be treated as "+X cards" for power level. It's different in several ways. You only get the draw 1 shuffle after you play the card, you can save Horses, but you don't always draw all your Horses. But very generally, it's basically +X cards.
Black Cat has an insane reaction that is held back by the terminal +2 Cards. You can definitely skip it, but if your deck can support Black Cat, it puts a lot of pressure on end-game VP.
Sleigh is similarly weak, where 2 Horses is just not great for a $2 cost (remember, it's roughly +2 Cards). The reaction can be nice but I haven't been impressed with buying Sleigh for just its reaction. If you're in the market for both the top and bottom of Sleigh, then you could try it.
Supplies is quite good. Some people say you should usually buy it over Silver. I'm not too far off from that. Topdecking a Horse is pretty nice and if you can draw deck then it's basically a $2 cost Peddler, except you get to frontload the draw to the start of your turn, which is good for reliability.
I have yet to get Camel Train to work for me. You're supposed to view it as a Duplicate, but one big benefit of Duplicate is that it sticks on your Tavern mat outside of your deck until you want to call it, and you can Duplicate Duchies. Camel Train just sits in your deck forever. I might be missing something but I find that it feels bad to play it before I have deck control, and by the time I have deck control I want to be spending my actions on other things.
Goatherd's fine. Sometimes skippable if there's no other trashing, +1 Action trash a card for no benefit isn't always good enough. But otherwise it's worth buying. Just remember the Goatherd draw isn't going to last forever.
Scrap's great, of the early Estate trashers it's one of my favorites. Unless you really need the +1 card now I think you should take the Horse over the card. I generally like turning my first Estate into Silver + Horse early, then the future Estates into card + action or horse + action. If you really need to, you can Scrap Silver --> Silver + action + buy to get a janky non-terminal +buy into your deck.
Sheepdog is another card that lives off its reaction, except you can actually trigger the reaction yourself. Can be inconsistent but the deck is powerful if when it triggers.
Snowy Village's +4 Actions effect is strong enough that I feel you normally buy one, even if you have other villages. If you have lots of cantrips, you think twice about it. Don't get more than 1, unless it's the only +Actions source, in which case you maybe get 2, but even then you want to get away with just 1.
Stockpile is insane, especially in 2 player. I'm still not sure how to play it, all I know is that if you play against someone who skips it you tend to win. My suspicion is that early on, you want to get all your Stockpiles off the mat before your reshuffle, but after you get a few, you only want some in the reshuffle.
Bounty Hunter is a nice thinner. (Or exiler? IDK what the terminology is). Makes it very easy to hit $5 and even $6 the first few times you play it. Usually buy 1, buying 2 can be reasonable, 3rd and up is a waste.
Cardinal is either great or awful and I feel it tends towards the "awful" side more often than not. It's not too rare to have a game where, say, your Village gets Exiled, but you just get it back immediately because you're looking to buy more Villages anyways. But when the attack is good, it's really annoying.
Cavalry's top portion is kinda eh. Its bottom portion is insane. Quite nice with trash-for-benefit.
Groom is great with Alt-VP rushes, and even outside Alt-VP rushes, Horse gains on your actions is good, and double Silver gains for money decks isn't bad either. In engine games it's already worth opening Workshop to gain $4 cost actions, Groom is the same thing except you get Horses along the way.
Hostelry would be fine even without the bottom text, the bottom text makes it interesting. It's basically "overpay for Horses". I will still take key $5s over Hostelry + Horses early, but later on I'm happy with overpaying for some Horses.
Village Green doesn't really need its reaction to be good, but if you have discard synergies, then yeah, go for it.
Barge should usually be played for your current turn, but if it's your last action and you have a decent buy in your current hand, playing for next turn is nice. And turns out a +3 cards +1 buy card is always welcome at $5.
Coven is weird. You can definitely outrace it, but if you get it wrong then you just lose. If you're trying to outrace Coven, skip Coven entirely, you don't want to help them get the Curses into your deck. If you're going for Coven, you want 2-3 as well as whatever other cycling you can get your hands on.
Displace is the standard Remodel tricks, except you also get to do Province --> Gold, Gold --> Province loops. Those loops are quite nice, because even if you don't fully loop the gains, turning a Province-in-deck into a Gold-in-deck (or another Displace) is already a good effect. You usually buy this card, there's something worth doing with it.
Falconer is conditionally good, you need good $2/$3/$4 costs to want it. Unlike Black Cat / Sheepdog, its base effect is good enough that you don't need to exploit the reaction a bunch to get your money's worth. But of course, if you can use the reaction, then you do so.
I've been disappointed by Gatekeeper, it just feels too slow to pick up a lot of the time. It's not Swamp Hag.
Hunting Lodge is the nuts. Like, dang, this card is great. You don't have to go full draw-to-X either, you just have to lean into the draw-to-X a little bit and otherwise just use the reaction when you draw it with a bad hand.
Kiln compares less favorably to Haggler IMO, but Haggler is a great card so that isn't saying much. I find that what happens is that you want to Kiln an Action card, but to do so, you need to go Village-Kiln-Village, or Village-Village-Kiln-DrawCard. It takes a while to get your deck to the point where you can Kiln Action cards, since your starting hand needs to have lots of +Actions in it. So you normally just Kiln Treasure cards...which is fine, free Gold gains are nice! But it's not quite what I hoped for out of the card. This is payload that you use after your deck is built to get more money into your deck, not a deck building accelerant to improve your draw.
By now everyone's praised Livery a bunch. When there's +Buy, Livery can really just turn into a Horse printer. Even without the crazy turns, you only need to gain 2 qualifying cards for Livery to become approximately +2 Cards +$3, which is a great deal for a $5 cost.
Mastermind takes a while to set up, but its effect is so good that you should get it early, it'll be worth it. Generally how Mastermind games play out is that you avoid putting Masterminds under Mastermind, until your last few turns, at which point you try to set up Mastermind-Mastermind-X-Y-Z chains and end the game.
Paddock is interesting. The base effect is close to the mythical +2 Cards +$2 for $5, a card that's very good in a money deck. In an engine deck, you still want Paddock, but you want to get an empty Supply pile fairly early to make it non-terminal. Once it's non-terminal, Paddock is insane.
The biggest difference between Sanctuary and its compatriots Junk Dealer / Upgrade, is that you can maintain a clean deck while greening by Exiling Provinces. Oh and the +Buy. Non-terminal +buy is always a win. I think Sanctuary is right around the same power level as Junk Dealer / Upgrade, they're all great cards and it's hard to go wrong with buying them as your first $5.
Fisherman is unexciting at $5 and you don't always have a good opportunity to buy it at $2, but you'll pick it up in both cases.
Destrier is better than Lab, which is a bit weird since Lab is already a really good card. If you can get +buy and draw then you get into situations where you're buying Destrier for $4 or $3, which is just absurd. You want to make deliberate moves towards getting cheap Destriers, because they snowball very rapidly - more Destriers = easier time drawing your gains or +buys = even more cheap Destrier.
Similarly, Wayfarer is insane - it's arguably the strongest in the set. (Or at least, if you argue it is, you won't be laughed out of the room.) It's just very easy to have some way of making the card cost less than $6, and if there isn't one, then you can pay $6 for your first Wayfarer and use the Silver gain to make your next ones cheaper. In Wayfarer games, focus more on draw. It's very easy to fill your payload space with free Silver gains off Wayfarer, so your aim is to make your payload space big (by getting lots of Wayfarers, wow this card does everything you want, wtf.)
And then there's Animal Fair, which is....eh, it's okay. Sure, it's a funny open with Necropolis. Otherwise, it's just kind of there. I'm not looking to go out of my way to get an Animal Fair, but I will pick one up if the opportunity shows up.