I don't like this card.

This is the S.H.I.E.L.D. team card, but its not really a team card... every single other team card buffs, or otherwise deals with allies:

Mighty Avengers makes allies stronger.
Guardians of the Galaxy gives card draw when you upgrade allies.
Uncanny X-Men gives allies more health and makes them cheaper.
Uncanny X-Force makes allies better at thwarting and gives them longevity.
The upcoming Flight Squadron team card for aerial gives ally readies when playing aerial events.

Everything makes allies better, and you're encouraged to play allies to make use of the team card.

Agents of S.H.I.E.L.D. does none of this. It's an encounter card mulligan. While good, it has nothing to do with allies. Furthermore, it discourages you from playing allies as there is no buff to be found for them, and having an ally that doesn't have the right trait will completely disable the card. Instead of giving you something by playing a tribal deck, this card takes it away by not playing one. In fact, you can play this card and use it while having 0 allies on the field, not very team-player of you.

The main reason I don't like this card is that it's taking the space of the S.H.I.E.L.D. team card and therefore we wont get another one. This card should've been something else, perhaps another location support or a title, or anything else other than the singular S.H.I.E.L.D. team card. The other team cards are awesome and lead to interesting strategies, this one has none of that.

But for the power of the card as-is, encounter mulligan is strong and welcome in leadership, I just wish it wasn't the team card.

I'd argue this is the most important card in Maria's kit. I'll often run Superpower Training just to help make sure I can get this down on the board. It's mostly the fuel that keeps her Helicarrier engine running, but I'll sometimes throw an extra counter on Med Team to help keep Nick Fury topped up (which winds up being another 2 counters) or Support Staff for another wild. Plus you can use it to basically stock up on counters since Maria can move a counter from one support to another each turn. Just overall an invaluable upgrade.

Extremely good card for Protection Thor (who becomes Aerial thanks to Mjolnir), or Spectrum in Gamma form, granting 2 threat removal plus at least 3 damage for 3 ER, an astonishingly good ratio (compare with Hit and Run which only grants 2 damage and 2 threat removal for 4 ER).

Obviously good for Angel as well, who draws a card when playing it, and for Nova, who relies on basic powers to generate resources from his Supernova Helmet, less so for Ironheart who, even if Aerial from the start, has lower basic stats and no particular need of readying herself often.

Many other characters can benefit from this card (Captain Marvel, Iron Man, Wasp, Star-Lord, Vision, War Machine, Valkyrie, Phoenix, Storm, Rogue, Magneto), although none of them is Aerial right from the start like the ones I mentioned above and thus have to rely on cards that make them become so. Personally, I wouldn't include Aerial cards in these characters' decks if I have to rely to a lucky draw, except maybe Iron Man and Magneto who have the means to tutor the cards that make them Aerial.

However, special mentions go to Doctor Strange, who gains Aerial from his Cloak of Levitation and loves to ready himself so he can cast more invocations, and Iceman who gets Aerial with his Ice Slide and loves to use his basic powers to attach more Frostbite to his enemies. If you play these characters with the Protection aspect, Ever Vigilant may be worth including.

Black Panther is an ally that is either too expensive, or suffers too much consequential damage: 4 ER (5 minus 1, the Event you tutor from the discard pile) for either 2 damage and a chump block, or 4 damage, is just a horrible ratio.

The only cost-effective use I can see for this ally is to play him with Sneak Attack, use his ability to tutor either a Go Down Swinging or a Save the Day card from your discard pile, make Black Panther attack or thwart, then use the very same card you've attached to him to discard Black Panther to deal 4 damage (with Go Down Swinging) or remove 4 threat (with Save the Day), since he would've been discarded anyway with Sneak Attack. Damage-wise, such a combo would deal 6 damage (2 for Black Panther's ATK, 4 for Go Down Swinging) for 3 ER (1 ER for himself, 2 ER for Sneak Attack; we do not count Go Down Swinging since it's tutored from your discard pile, as explained above). 2 damage for 1 ER is good, but it's a very clunky combo which requires your deck to be built around it, with lots of cards required just at the right time or in the right place... and you're also wasting a block. Just not worth it.

I simply house-rule him with 1 less consequential damage for both ATK and THW. Otherwise, I don't see any reason to include Black Panther in any of your decks, especially with so many better options out there (especially in Leadership).

Now that so many heroes have come out, I find it even more baffling that this is the only threat removal card in Spider-Man's deck. The threat/ER ratio is 1.5, which would have been good... if only it weren't conditional. The immediate comparison is Chase Them Down, which offers a hefty threat/ER ratio of 2 basically at the same conditions (of course there are hidden factors, like the fact that Spider-Tracer is an attachment so it offers more flexibility, but still I don't think it's worth the lower ratio).

All in all, I think this card is quite mediocre and depends too much on the presence of minions. What's even worse is that this card alone makes Spider-Man almost unplayable in Solo with aspects other than Justice, unless against minion-heavy encounter decks, which is honestly quite a shame.

I tend to house-rule that this card is still an upgrade but it doesn't need to be attached to a minion. Instead, the text reads as "Action: Discard this card --> remove 2 threat from a scheme." I find it much more enjoyable and flexible, and it makes Spider-Man more playable with different aspects.