This question is about the Revised Ranger from Unearthed Arcana, not the one from the PHB.
In the new Natural Explorer feat for the Revised Ranger it says
On your first turn during combat, you have advantage on attack rolls against creatures that have not yet acted.
That means apparently that you always have advantage at your first turn against everyone you beat in the Initiative order.
But what if you multi-class with rogue and have access to Sneak Attack? Would the advantage from Natural Explorer trigger Sneak Attack?