Shadow of a Doubt

My take on this set: I am not particularly worried when i need to shuffle this set into the encounter deck. Knives in the Dark can be an issue for some investigators, but is very context dependent. Undercover is just filler.
This set does a fine job of giving the concealment mechanic a bit more depth, but will rarely stand out as particularly impactful. That is perfectly fine, of course.

What it does: Knives in the Dark enters a players threat area and stays there until they pass an agility test at the end of their turn. While affected by Knives in the Dark, the player will suffer 2 damage whenever they expose a decoy. Drawing this card while already having the other copy in the threat area will make it surge instead.

My take: Pretty terrible if you are the one who is doing most of the exposing but the more players are in your team, the easier it is to circumvent. There are a lot of damage sources in encounter decks throughout the Scarlet Keys, so there’s definitely a good potential for Knives in the Dark to stack up in an unpleasant way.
Due to its timing, which follows such classics as Frozen in Fear, this is going to take effect for at least one turn and it’s somewhat hard to get rid of. At least it’s not as debilitating as Frozen in Fear is, i guess.
This is pretty rotten to get on a fragile low agility character like Daisy or Dexter and can hinder them from safely exposing concealed cards for several turns.

Threat level: Medium. Absolutely fine on its own, but if it becomes part of critical mass of damage sources, it can be painful. Mid to High in solo where you have fewer options to play around it.

Dealing with it: The “at the end of your turn, test” timing is about as bad as it gets. You only get one chance to discard the thing each turn and other players can’t discard it for you. If you are stuck with this and can’t offload the exposing duties on someone else, you might just need to get lucky or have other ways to mitigate damage.

What it does: Following a failed intellect or agility test, an additional decoy is shuffled into the concealed mini-cards at the location with the most of them. If no enemies are in the shadows, Undercover surges.

My take: This is fine. It can potentially waste an action, but that’s totally what we expect from an encounter card anyways. As long as you don’t have to exert great effort when exposing cards, having to go through another decoy won’t slow you down much.

Threat level: Low. Not a very impactful card, in fact it might just do nothing at all.

Dealing with it: No need to deal with it specifically. Just keep exposing those mini-cards and it will sort itself out.