Will add some pieces of data I was looking for when entered this comment section and didn't find it fully. I will repeat something of what was already mentioned, trying to make my own review pretty comprehensive. Everything turned out to be in the rules, though, but who reads them before they need it? DISCLAIMER: I will be using some encounter cards from different cycles as examples, so if you care about that sort of spoilers, don't follow my links).

• What is "skill test from an ability"? I was sure that ability is only what is triggered, but I was wrong. Yes, there are triggered abilities (if a scenario card has any type of arrow, triggering which you initiate a skill test, you can commit this card, e.g. Bedeviled), but also: constant abilities (an effect that simply exists and doesn't have any specific point of initiation according to the rules. As I understand this card fits), forced abilities (something after word "Forced" that initiates a skilltest. In theory, bc I didn't find examples) and revelation abilities (skill test after word "Revelation", so, yes, basically half or even more cards of the encounter deck, like our favorite Rotting Remains or Crypt Chill). That's when I got true understanding what is this card about.
• What is "scenario card?" From rules: "Scenario cards include act cards, agenda cards, location cards, treachery cards, enemy cards, and scenario reference cards." and "Weaknesses with an encounter cardtype (such as enemies or treacheries) are considered to be player cards while they are in their bearer’s deck, and are considered to be encounter cards while they are being resolved, and once they have entered play". So you can find some clues while committing True Understanding to Searching for Izzie. And don't forget that abilities on weaknesses in threat area or attached to the location can be triggered by other investigators in the same location (yes, pure Jenny can spare her out-of-class card room for something more appealing while her seeker can watch her back). Above mentioned Bedeviled now can also be taken away from Finn with some additional use by your seeker. And, basically, any parley action with skill test can be assisted with True Understanding. This chatty Cathy Poltergeist! Always spills the beans.
• TL;DR the card is not that situational as one can find and, given you can commit it not only to your own skill tests, you are unlikely to find yourself in a situation when it's been uselessly sitting in your hand for half of a game without any chance to be triggered. It's much more easier to use it than, for instance, Evidence! while it is an actionless and 0-resource cost way to get a clue. The test, you need to pass, can also be easier than the location's shroud value.

A straightforward use for this is as a curse-heavy solo Rogue, at which point this simply turns half your curses into blesses. In theory, this averages the value of all curses to 0.

In practice, of course, you still have increased standard deviation, which is worse than not having the curses there at all... your worst-case scenario still means assuming all your curses come up cursey.

So perhaps the better way to look at it is as an automatic 50% saving-throw-vs-curses. Difficult to say if that's worth a 4xp one-of.

I have run this a couple of times with parallel Roland now, because one of his directives (Red Tape) makes Guidance fast! Now it may still not be the most optimal card, but now you don't lose an action playing it, which is good.

I think this card can be situational (for 1 data point, I had fairly little use for it in my recent Carcosa/2-player campaign), but it had strong synergy with Zoey Samaras for specific spectacular situations, mostly because of 2 things:

• it’s a nice counter (esp. updated version) to her fairly painful signature weakness
• because it’s situational, you can put it under Stick to the Plan (which she would usually want to run anyway) in place of an e.g. Emergency Cache (which she needs less of) in the usual “Ever Vigilant - Money - Prepared for the Worst” builds. This way it is never a dead draw, which it is bound to be sometimes if from your deck. (This is also part of the value of having Prepared for the Worst as part of the plan)

I’m mostly sad that it and it’s updated version can’t do anything about SPOILER, otherwise it would have scorpioned two potential birds with one stone.

Definitely a good card for Nathaniel to take, but I could see uses for the card in other investigators. The engage part means you can attack enemies on another investigator without accidentally hitting them, and you can drag Aloof enemies to you. Fast means you can play it on another investigator's turn, so you don't have to go first just to take care of a monster engaged with them. Moving the enemy means you don't have to spend your actions to get to the enemy's location. Plus, if your deck has cards like Evidence or Grete Wagner (or if you're Roland), you can reposition an enemy onto a location with an otherwise hard-to-get clue.

Not a must-have card for every character, but in larger player counts (where monsters are more likely to show up and there's more non-fighting allies you might want to protect), in campaigns with more Aloof enemies (hello, Whippoorwills), or if you have reasons to want enemies in specific locations, it's probably worth checking out.