This card gets better, when you know the scenario and "ambush" fixed spawns (advancing act or agenda).

It doesn't matter which player stays behind. This allows the parties fighter to "protect" a clue gatherer, as long as the gatherer stays at same location.

Even if it does not trigger, it may safe a lot of time if the gatherer knows he is safe for 1-2 rounds while the fighter saves someone else or hunts specific enemy (Smite the Wicked).

It's very useful when there's lots of clues on one location, like in bigger parties (3-4 players) or special locations (The Hidden Chamber).

It also helps on higher difficulties, as the damage is an "auto success".

Just a reminder: On the Hunt and "Let me handle this!" exist and are good cards (especially together as a powerful "enemy funnel" build). The fact that these cards exist means that you can ensure that enemies get delivered to your doorstep to trigger this trap. On the Hunt especially is a powerful card for this purpose since it overrides normal spawn rules to put the victim not on its normal spot but right on top of you and also shortens the time you need to wait and stick around for a foe to appear.

Ideally you'll wait for both cards (Ambush and On the Hunt) to make it to your hand, then you play them in a combo.

.

Spoiler alert for some relatively common (first scenario per story) enemy cards

2 hp foes like Ghoul Minion and Fanatic just blow up.

3 hp foes like Ravenous Ghoul are now firmly in boxing reach (With or without Mano a Mano.

Most importantly, 4hp foes like Yithian Observer, Agent of the King and Icy Ghoul are now one-shottable with a gun and firmly within striking distance.

Situational card just like Barricade. Can be used when you are preparing for a boss fight but other than that i can't see any play. Bad card for Solo since you need to move fast from location to another most of times. Can be utilized in O'toole decks with sneak attack combo for a total 4 dmg without even fight (let's add a dynamite blast there going up to 7). Other than that Harrigan and Yorick have better options for 1xp than this one.

A group, meta card. If your group is running any combination of Alyssa Graham, On the Hunt, Split the Angle and/or Scrying, this card is really, really good as you will always know exactly when to use it.

And at 1 XP, it is worth the buy in those situations.

Outside of it, I would probably only run 1 in a On the Hunt Guardian (aka almost everyone everyone who can run guardian). If nothing else, you pay 2 and 2 to save you an action on your next turn.

The main problem is that you cannot be sure that an enemy is spawned at your location in mythos phase. If an enemy is not spawned, you have to plan that you don't move one more round, or discard 2 resources and Ambush; both are bad. On the Hunt & Let me handle this! may help this work, but both card don't guarantee that a enemy is always spawned.

However, it is another source of damage in mythos phase for Nathaniel. Because Nathaniel has very limited cards which can deal damage in mythos phase, this card always guarantees 3 damages without any loss. The fundamental problem can be solved by Gloria. Gloria can look encounter deck. If Gloria looked encounter deck, you can be sure that an enemy must be spawned. Then, it always works! Also, Gloria usually wants to look encounter deck, this has no additional cost like On the Hunt and Let me handle this.

I think Ambush is good card for Nathaniel & Gloria pair. Also, it's good even if only Gloria exists (of Gloria herself).