Spiders

My take on this set: Unless the players themselves suffer from Arachnophobia (in which case, what are you doing? Play TFA or something), the cards from this set are unlikely to phase any seasoned Arkham player. While the cards on their own are somewhat unassuming, there are some synergies on other scenario cards that can change this. The second half of Waking Nightmare gives each spider a +1 boost to evade and fight. While unlikely to matter for the Swarm of Spiders, it does matter a lot for Spider of Leng. The difference between 3 and 4 in a test difficulty is quite big. In the same scenario, the infestation tests will also keep a steady flow of spiders coming. While a Swarm of Spiders requiring two of your actions is usually not too bad, it quickly can become a problem if that happens on top of whatever the mythos phase reveals.
Also, the Spiders set always comes together with the Agents of Atlach-Nacha set, which has a nasty treachery that becomes a lot harder to evade when spiders are around.

What it does: Swarm of Spiders is a swarming enemy. On its own, it’s just a 1/1/1 that deals a point of damage, but the two swarm cards it starts with act like extra health and they also each increase its evade.

My take: For most purposes, Swarm of Spiders is just like a Swarm of Rats from the core set, but with three health. This gives it some staying power and if players are unwilling to spend a full turn in melee to kill them, they will at the very least make the players spend some cards or charges on it that would otherwise have been available for more immediately threatening cards.

Threat level: Low. Dealing with them is easy, but will take some sort of resources away, be it cards, charges or extra actions.

Dealing with it: The two scenarios that have the spiders in them aren’t really ones where you want to spend three actions on punching some enemy to death, so using some sort of three damage attack is going to be the best way to deal with these. You can of course evade them, but both scenarios feature at least some backtracking, so that can backfire later on… especially if a Spider of Leng gets involved and adds more swarm cards to the pile until evasion even stops being an option.

What it does: A monstrous spider with four health which puts it well out of most one-shot kills. In addition to having basic combat stats of three fight and evade, the Spider of Leng also creates more Swarm of Spiders cards which are pulled from the encounter deck and discard if necessary.

My take: The stats are nothing an investigator that is prepared for a fight can’t handle, but the high health means that it will almost always take several actions to get rid of this enemy. The Forced ability makes evading the Spider of Leng a very unattractive option unless you know that you are not going to ever go to that location again as it will over time create a huge swarm at its location (using player cards, which is a concern for some characters as well).

Threat level: Mid. The Forced ability makes it hard to ignore, so you end up having to spend some actions on killing the thing.

Dealing with it: As far as enemies go, the Spider of Leng isn’t anything too special to handle. Just shoot it, slash it, burn it, whatever it is you usually do with enemies.

What it does: Sickening Web attaches to a location until an investigator uses an action and passes a fight or evade test at difficulty 3. Until then, investigators can not leave the attached location and all Spider traited enemies there gain Retaliate and Alert.

My take: At this point, we’ve seen this sort of treachery template many times and as with many of the other similar cards the impact depends a lot on who is drawing the card. Most investigators should be able to pass one of the two tests, but some exceptions like Norman may need bailing out by another player. The restriction to movement is the most relevant effect of this card, the bonus abilities for spiders rarely matter. Grey Weaver from the Agents of Atlach-Nacha set and of course Atlach-Nacha herself are the only spiders where you would usually expect to fail a few tests (outside of the ever-present autofail) on attacking or evading, the rest all have stats like the Spider of Leng or lower.

Threat level: Low to Mid. Occasionally annoying, but can often be discarded with just one action.

Dealing with it: If you find yourself engaged with a spider enemy while drawing this, note that taking the action to discard the treachery will provoke an attack of opportunity. So usually, you’ll want to get rid of the creature first.
Unlike many other similar treacheries that came before it (like Locked Doors and Overgrowth) this one has difficulty 3 on the tests instead of 4. This opens them up to a lot more investigators.