City of Sins

My take on this set: This set is interesting to me because both cards are very different and yet they synergize with each other well due to how they work mechanically. Centuries of Secrets can, depending on the investigators in play, be a very important card to play around. There’s three of them in the deck, so expect to draw them often. Evil Past is much less scary, and mostly filler to up the count of Curses in the encounter deck for Centuries to trigger off.
In my opinion, discarding cards from the encounter deck in itself is a more relevant effect than discarding cards from player decks. It leads to the deck depleting quicker, which in turns leads to a reshuffle of the encounter cards. Quicker turnarounds of the encounter deck make it less predictable, possible to see particular cards a lot more than one would assume and ultimately feeds into itself because you are just going to redraw cards like Centuries of Secrets much quicker. I think this is a more relevant thing than the one point of horror attached to a player deck reshuffle.

What it does: Depending on how well (or badly) the player does on a Willpower check, cards from the encounter deck are discarded. If a Curse treachery is among those cards, that player’s investigator and all of their allies are dealt one direct damage.

My take: A very potent card that changes the rules just for existing in a scenario. Not only is direct damage a problem for some investigators like Calvin or Daisy, but especially the damage to allies is a huge deal for many. The difficulty of the Willpower test is high enough that even Mystics will often have to discard a few cards to this, potentially triggering it and losing their Arcane Initiate or similar.

Threat level: High. This is one of the cards that, just for being present in the encounter deck, have a sizeable impact on how players can use their cards. There are only few allies around that are immediately defeated from one damage (Arcane Initiate and Peter Sylvestre are the two most important ones), but this card can easily be drawn multiple times and even if not it limits players in how they can use their allies for soak if they don’t want to risk losing them.

Dealing with it: Know it’s there and think about it when placing damage on your important assets. Investigators with higher health are much less threatened by this cards because they have a buffer to work with, while someone like Sefina will struggle a lot. For those characters with few health points to work with, consider investing into non-ally health soak like Leather Coat or even Bulletproof Vest. The Willpower check is usually not worth pitching cards to, the difference between discarding 2 or 4 cards is not that large due to only one Curse card already triggering the full effect.
A final note on Curse cards: The only other Curse treachery in the non-scenario sets is Diabolic Voices from the Witchcraft set, but there are extra Curses in Secret Name, Wages of Sin and Clutches of Chaos.

What it does: Evil Past attaches to a player, doing nothing until the encounter deck runs out. Once that happens, the investigator is dealt two horror and is allowed a Willpower check to discard the treachery. Should the test fail, Evil Past sticks around to trigger again once the deck runs out at a later point in time.

My take: No initial impact means that this card offers a bit of breathing room to deal with lingering threats from previous rounds and/or to progress towards the scenario goals. There’s not much to be done about this card, it will just deal some horror eventually. While it’s one of the few instances where the damage happens without any saving throw, this can easily be considered a lucky break when drawn.

Threat level: Low. Being dealt two horror isn’t a large effect in the bigger picture in the first place and this card usually offers some time to prepare for it. The repeatable part doesn’t matter all that much usually either. And again, if it should matter, there is time to prepare for the Willpower test by saving up a card or two to pitch for icons. The threat level goes up when playing with 3 or 4 players due to depleting the encounter deck faster, but in the grand scheme of things it should still be more than fine.

Dealing with it: Grin and bear it. The first two horror will happen, not much to be done about it. If already at low sanity, then digging for some asset to soak can be a consideration, but that should be the exception.

Return to Circle Undone: City of the Damned

My take on this replacement set: I like it, mostly on the back of Unhallowed Land. Removing Centuries of Secrets from the picture which is very punishing towards certain deck types is a good thing, and so is replacing a couple willpower tests with agility. Vice and Villainy is a bit low impact, but i wasn’t terribly impressed with Evil Past either. Either card can really only get out of hand in full groups where the encounter deck cycles a lot faster than in the two-handed games that i am used to.
I’d be interested in mixing and matching the new and the original set. Playing with 1 Evil Past, 1 V&V and a random set of three from Unhallowed Land and CoS would be spicy indeed.

What it does: Following a difficult agility test, the investigator has to discard a card from the top of the encounter deck for each point they failed by. If a Curse treachery is discarded this way, a direct horror is dealt to the investigator and each of their allies.

My take: Unhallowed Land closely mirrors the original Centuries of Secrets which it replaces. Instead of a willpower test, there’s an agility test and instead of a damage it deals a horror to the affected targets if a Curse gets shaved off the top of the deck. Switching damage for horror makes a big difference here, at the time of writing this there are only 5 allies in the game that die to this but survive a Centuries of Secrets. On the other hand, there are 24 allies that die to Centuries but survive Unhallowed Land. This immediately makes this treachery a lot less oppressing than Centuries, even if the agility test is arguably more difficult than a willpower test is due to players being less prepared for it in TCU.
Of course, this card still keeps Anna Kaslow from the board, which is a travesty considering that TCU would be the campaign where i’d thematically want to do a proper Tarot centric deck.

Threat level: Low to Mid. If you aren’t losing an asset to this, it’s actually fine.

Dealing with it: Players that run a host of allies at the same time (like your typical Leo or Tommy deck) will not be too happy to see Unhallowed Land, but everyone else is going to sigh a breath of relief that Centuries of Secrets left the encounter deck. It’s still a direct horror to the player and some stress on the soaking assets, so be aware of that should you be vulnerable to that due to low initial sanity or having already taken a few hits.

What it does: The investigator attaches this card to one of their assets. Whenever the encounter deck runs out of cards, the attached asset is discarded. If the asset is discarded to this effect or for any other reason, Vice and Villainy has to be attached to another asset unless the player passes a willpower test.

My take: This seems rather soft to me. This directly replaces Evil Past, substituting the loss of 2 sanity with the loss of an asset of the player’s choice. That seems to me like it’s usually in the player’s favor, especially as it won’t discard the asset immediately like a Crypt Chill (or similar) would. So, the player has time to spend all charges/ammo/secrets from that asset before it gets discarded. The only drawback is that Vice and Villainy can trigger before the encounter deck runs out too, as the Forced effect will fire when the asset is discarded for any reason. That could potentially be annoying for someone like Dexter or Yorick, but can be interrupted by passing the willpower test.

Threat level: Low. No immediate impact and even the delayed payoff isn’t all that impressive.

Dealing with it: Depending on how confident you are on passing the willpower test, you might want to hold off on playing another asset into the same slot as the one that has this treachery attached, just to keep the amount of reattachment triggers to a minimum.
(Edit: As pointed out by commenter Whimsical below, the upgraded Mag Glasses can be used to defang Vice and Villainy completely by returning and replaying it in the right player windows. Might be of interest if you don’t mind a little bit of cheese with your card play.)