Pros:

1. Strong signature cards (especially Dark Insight)
2. Decent intellect and strength
3. +1 resource/ +1 card activated effect is strong incentive to play ignore/cancel effects, since it often makes them free cantrips. This also makes her deck cycle faster.
4. Access to cards, which increases her potential for combat
5. Potential for 6 base willpower in late game
6. She starts with 6 cards (with Dark Insight as one of them), and if you mulligan you can get up to 6 new cards!
7. I think her signature weakness is not as devastating as many others. You can choose which cards to ditch and which to keep for 1 horror. This actually helps clear space for new additions. But it also favors not relying too heavily on her willpower (see below).

Cons:

1. Very low starting willpower. This creates a conundrum because cards like Shrivelling will be very difficult to use for part of the game, and hard to justify with access to cards like Machete. Upgraded spells that boost willpower should be easier to use. If you do rely heavily on willpower, it incentivizes playing your cancel/ignore effects earlier, and even potentially wasting them instead of saving them for harsher threats.
2. Why choose Diana Stanley to wield Machete when you can use other guardians with 4 or 5 strength? I think you have to make good use of her cards and 3 intellect to justify this.

When I think about cultists in this game, I think about not tremendously tough, not tremendously damaging enemies that treacheries can turn into absolute landmines. The longer they’re around, the tougher they are to fight and the more doom they bring to the table. Diana's the opposite.

Diana, like Gloria, represents a class of mystics that no other class can really replicate. As mystics with guardian access, they can help the party survive with sheer denial. (It’s why both of them are probably bad in improv.) Miss Stanley, the investigator you probably care more about because you’re reading about her, doesn’t really fit into the clue seeker/flexible/fighter archetypes. She can do any of those things, but more than that she can simply ignore bad things that are going to happen. She can play defense in an amazing way.

In fact, putting Diane on the team makes more risky characters viable. Low willpower rogues like her cult buddy Preston, oddities like Calvin...They can spread their wings, knowing that the worst of the encounter deck will be taken care of for them, and that their weakness can be ignored at least once due to dark insight.

Diana might be a bit of a one-trick pony, but it's a really good trick.

Diana's stats and deckbuilding options are just mediocre, so we're playing her for her unique abilities, all of which are stellar.

• Whenever one of her cards cancels or ignores an effect, if she doesn't have 5 cards under her, she gains a card and a resource. It's quite easy to do this 6 or more times a game (yes, even with the 5-card limit). You will be bringing cards like Ward of Protection, Dark Prophecy, and Deny Existence, which are already good enough on their own, but with Diana they become virtually free. s can sometimes struggle to have enough cards and resources, but Diana is getting paid to shrug off Mythos cards.
• Her gets boosted by +1 for each card beneath her, allowing her to get up to 6 without assets in play. That said, you might not be at the full 6 most turns, but Diana can operate just fine without requiring max . If you do want to go the max route, and are the classes to do it in.
  • This does means her starts at 1; Diana is not prone to explosive starts. But it's not hard to get up to 4 within 2 or 3 turns, thanks to...
• Dark Insight always starts as a 6th card in your opening hand. It's a great card on its own, it's even better with Diana's ability, and you will always have it during the first Mythos phase if you aren't done setting up your kit and need to keep the monsters away for another turn.
• Her other unique card, Twilight Blade, lets you replay cards you've placed beneath Diana. This can be a bonkers effect in some decks, as you're getting multiple uses out of your cancel cards, but also freeing up space under Diana so you can continue to get your bonus card and resource from future cancel cards. You do have to pay full price when playing cards this way (no bonuses), so if you want to build a deck where you cancel 15 things a game, bring ways to pay for it. And I guess you can stab with the blade too, if that interests you.
  • Her gives a similar effect, but at random. It's usually a nice bonus when it happens.
  • Oh, and her weakness? You can't cancel it, but you can ignore it with Deny Existence. You can even intentionally peel off a card or two to make room for more stuff.

Alright, so she's great at making nothing happen, but what does she do to advance the act? You've got the full suite of cards, so you can pick a lane between cluever (Read the Signs, Sixth Sense, Rite of Seeking, etc.) and fighter (Shrivelling, Azure Flame, etc.). Cyclopean Hammer makes her a reliable damage dealer if you go that route; Bandolier lets you also hold your Twilight Blade and boost . "I've had worse…" lets you get paid to tank, and works with Diana's cancel ability. Sword Cane can be a fine starter card until you get some XP. Arcane Initiate can find spells (which many of your "cancel cards" are, further fueling your card draw and boost). Prophetic and Robes of Endless Night can save you resources as you play all your spells. And Dayana Esperence can be a nice "win-more" card since most of your cancel cards are spell events.

Diana's a lot of fun... you start out slow, but you can "powerup" while protecting yourself from early Mythos cards, then start steamrolling as you get your core assets out and have a nice high . And near the end of the scenario, you bring out your Twilight Blade to replay your bank of cards under Diana and provide extra protection against the Mythos deck to make sure your team doesn't trip at the finish line.