This question is about this UA alternative ranger.

Ambuscade, by its meaning, is basically an ambush. If two characters, Alice and Bob, are interacting peacefully, and Alice suddenly attacks Bob, Bob does not count as surprised (as I understand it. Correct me if I'm wrong). However, what if Alice is a Ranger (UA Alternative Ranger) with Ambuscade? Would this mean that Alice is suddenly "ambushing" Bob out of nowhere? Or would Bob still count as unsurprised because he was aware of Alice's presence?

Now, if Andy was being stealthy and went unnoticed by Ben, and suddenly attacks Ben, Ben is definitely Surprised. If Andy is a ranger with Ambuscade, when does the surprise take place? Does Ben count as surprised during Andy's special turn with Ambuscade, or does he count as surprised in the normal turn after Ambuscade, or both?

The reason I am asking this is for an Assassin/Ranger multi-classed character. As Assassin, the character needs to have the enemy surprised for an automatic critical hit upon hit. I am wondering how the character's Ambuscade feature (as a Ranger) affects this.

I am asking this from the perspective of a player, not a DM. I am not looking for answers that start with "As DM, I would...". I am looking for answers that are based on real sources (including UA) and written rules.

I know that nothing about the Ambuscade feature itself says it affects surprise (of the enemy). I am looking for citations of rules about Surprise itself that would give clues as to whether or not Ambuscade would logically affect it. For example, a feature that guarantees a Stealth check success would not state that it affects surprise, yet it does.

Ambuscade doesn't affect surprise.

The key to this is in the wording of the class features. The Assassin Rogue's Assassinate feature says "any hit you score against a creature that is surprised is a critical hit" (p.97). In the section on Surprise (p.189) it says "The DM determines who might be surprised", with some guidance on what might count as surprise.

Ambuscade only lets you make an extra attack at the start of a fight, it doesn't affect surprise at all. If you surprise you enemy, then your Ambuscade attack as well as your surprise round attack both automatically crit, since the target is surprised until the end of their first turn. If you don't surprise your enemy, then your Ambuscade never gives you an automatic crit.

Aside from the automatic crit, any attacks made with Ambuscade have advantage, since they are necessarily made before your enemy takes a turn.

Ambuscade was removed for a reason.

The Ambuscade feature comes from an earlier revision of the UA Ranger. The latest version (currently found here) does not have this feature at all. In playtesting, it was found that single-class rangers didn't have a problem with this feature, but multiclass rangers were much more powerful when they had this feature, so it was removed entirely from the class.

I would heavily suggest using the more recent version of the UA ranger, rather than the older one.

Ambuscade is not a surprise round

Ambuscade states:

When you roll initiative, you gain a special turn that takes place before other creatures can act. On this turn, you can use your action to take either the Attack or Hide action.

While this is similar to a surprise mechanic, it is different and they discuss how it works if you are surprised:

If you would normally be surprised at the start of an encounter, you are not surprised but you do not gain this extra turn.

UA is not tuned for multiclassing

Just a reminder that UA is not tuned to multiclassing and interactions between features often aren't balanced. In this case, it Seems like they did consider someone wanting to do this, which makes sense. Assassinate is very powerful and a class fetaure that would give you that mechanic at the start of every fight would be OP.