When reviewing a card, one can't help to compare that card to other cards that do similar stuff. The "classic" healing options mostly are assets that use charges and require comming available actions to actually heal you. Compared to earlier in- and out of faction healing options, Emergency Aid offers something new. Well, 2 new things.

First, it let's you heal allies, which is nice . Second, it's an event, which means you commit 1 action to it instead of commiting an action to play it and then use more actions for each point you want to heal. When compared with the classic Guardian healing option First Aid, it becomes clear that Emergency Aid is a more "action efficiënt" way to heal. With Emergency Aid you pay 2 resource and use 1 action to heal you for 2 points, while with First Aid you pay the same amount of resources but have to use 3 actions to acually heal you for 2 points (1 for playing the asset and 1 action for every point of healing.)

Does that make Emergency Aid better? It al depends on your investigator and the deck you're playing. When used with a deck that is very "action heavy" or when you desperatly want to keep an important ally alive, you might like a healing option that is more action-efficiënt and heals allies. But when you're deck is less action heavy or you're playing an investigator with low sanity, you might prefer the ability to both heal physical damage and horror.

I really like these little nuances the designers are making in the different healing options and those make for very interesing deck building choices.

How many allies can benefit from the full heal (AKA, how many allies have 3+ health)? nine (as of The Search For Kadath, and excluding Jake Williams).

Of these options, Beat Cop (2), Guard Dog and Agency Backup are the ones that most synergies with Emergency Aid. Trusted can push some allies into the three health range, and of course all investigators have enough health to use the full effect.

But really, you two main uses for this card are in an ally focused deck, or in a Mark Harrigan deck to prevent the bad side of Sophie. Take Second Wind instead for all other cases; cheaper and with free card replenishment!