Although the Rogue/Assassin's Assassinate feature and the UA Revised Ranger's Natural Explorer both grant advantage on certain attack rolls early in combat, the wording of the condition that grants advantage is slightly different:
Assassinate (PH p. 97)
… You have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet.
Natural Explorer (UA Revised Ranger p. 3)
On your first turn during combat, you have advantage on attack rolls against creatures that have not yet acted.
A creature that is surprised cannot move or take an action during their first turn. If they win initiative against an Assassin, it seems the Assassinate class feature does not apply, because the creature has "taken a turn" doing nothing. OTH, against a Ranger, it seems that they "have not yet acted", so the Natural Explorer feature does apply.
How (else) do these features operate differently, if they do?