Miasma

My take on this set: Miasma is a set that takes over for all the weather themed stuff from the first half of the campaign, to still provide some location based treacheries once the players are no longer exposed to the antarctic colds. Both of these cards make an effort to come after the health and sanity of your partner assets, so they do at least have some urgency to them and can’t be ignored.
Their actual impact is debatable, though. If you are willing to take some risks and don’t sweat the occasional damage/horror token on your partners, these cards shouldn’t be much of a concern to you. If you have healing available, they are downright trivial. If you want to play super safe however, you will need to make time and spend actions on discarding Torrent immediately or moving out of the way of the Miasma. That’s still not too taxing though when compared to what else usually lurks in the encounter deck.

What it does: Miasmatic Torrent attaches to a partner asset controlled by the investigator, exhausting the asset in the process. While the treachery is attached, the asset can not ready and is dealt either a damage or a horror at the end of each turn.
To discard the card, the player has to take an action and pass either a Willpower or an Intellect test.
If the player controls no parter asset when drawing this card, it gains surge.

My take: Thankfully you get to choose on whether to deal a damage or a horror when this card triggers. This card alone is usually not going to threaten to kill your partner asset, but it can absolutely contribute to making it more vulnerable to either Nebulous Miasma or Polar Vortex.
Getting rid of the card isn’t terribly difficult, but it does cost an action. I am rather paranoid when it comes to the well being of my partner assets, so i usually try to get rid of this card as soon as possible, but if you do have healing available in your deck you might even decide to have it stick around a bit while you care about more pressing matters.
It depends on the ability that gets blocked as well, of course. If you depend on Eliyah for emergency evades, you will want to clear this immediately. If you brought Tanaka to help with money, it’s probably less of a priority.

Threat level: Low to Mid. The partner assets do have a good amount of combined health and sanity to take this for a turn or three.

Dealing with it: If you do use your partner’s health and sanity to soak damage and horror for yourself aggressively, this card becomes a much bigger issue. During the scenarios where Miasma is part of the encounter deck, you might want to hold back on that a bit and make sure that your partners can survive this card (and Nebulous Miasma).

What it does: Nebulous Miasma attaches to the location of the investigator who drew the card. It is automatically discarded at the end of the round. Anyone ending their turn at its location is dealt a direct horror to each of their cards with sanity. This includes the investigator card and story allies. .

My take: During the first half of the campaign, the Deadly Weather set is everywhere and with it, the Polar Vortex card which goes after the stamina of players and more importantly, of the partners. Once past the Forbidden Peaks, that set disappears completely and we instead have to deal with Miasma and therefore this card. Nebulous Miasma is an exact mirror of Polar Vortex, only it attacks sanity instead of stamina.
In the direct comparison between the two cards, there are two reasons why Polar Vortex is a much stronger card, though. The first reason is Forbidden Peaks, which dials up the danger on location based treacheries (like Vortex) through its linear setup. The scenarios that Miasma is involved in do not have such restricted movement, making it more easy to evade. The other reason lies in the affected partner assets. The two low stamina assets threatened by Vortex are Danforth and Dyer, both quite important and powerful. On the other hand, the two low sanity assets threatened by Miasma are Ellsworth and Cookie. Ellsworth can actually protect himself through his own ability. And Cookie… personally i would call him the most expendable member of the expedition, but of course your mileage may vary.
Basically, i don’t feel nearly as threatened by Nebulous Miasma as i do by Polar Vortex.

Threat level: Low. Can usually just be sidestepped. That can take an action from you, but that’s just par for the course for a treachery.

Dealing with it: It’s going to be very rare that you’ll run into situations where you aren’t able to move and where the loss of sanity is hugely impacting. In most cases, you can just either take it or invest an action or two to move away.