What is the difference between a unit with Reaction keyword and Ambush keyword?

The core difference is that Reaction gives only timing permission, while Ambush gives both location permission and conditional timing permission.

Here's the breakdown:

A unit with [Reaction] (Rule 813):

• Can be played during Closed States on any player's turn (Reaction timing).
• Still follows normal Unit placement restrictions — it can only go to your Base or a Battlefield you control (Rule 813.3.a). You cannot Reaction-play a unit to a contested or opponent-controlled field.

A unit with Ambush (Rule 822):

• Grants two separate things (Rule 822.1.b):
  1. New location: "I may be played to a battlefield where you control Units" — this lets you play it to a contested battlefield where you have units, even if you don't control that battlefield.
  2. Conditional Reaction: "I have [Reaction] as long as I'm being played to a battlefield where you control Units."

So the practical differences are:

• Locations: Ambush can reach contested battlefields you're fighting at; Reaction cannot.
• Base play: A Reaction unit can be played to your Base at Reaction timing. An Ambush unit can be played to your Base only at normal [Action] timing — the Reaction is conditional and doesn't apply there (FAQ #10089).
• Timing consistency: Reaction works unconditionally. Ambush only gives you Reaction timing when playing to a battlefield where you already have units — if you play it to your Base or to an empty field, it's [Action] timing only.