Actually, there are several ST ambush cards that can be both loyal and disloyal, like Ciaran and Toruviel (at least they used to be, I haven't checked after the last update).
Theoretically you can put Toruviel on the opponent's row and pass to counter Villentretenmerth, but that seems like a very rare opportunity.
So yes, Trapper is like the only ambush card that is played on the opponent's side, so it's pretty obvious. The card would be more viable if it was changed to smth like this:
Ambush: When a revealed non-Gold unit appears on the opposing row, move to that row and remove 4 strength from all non-Gold units on the row (except self).
But that could make it an autoinclude in all ST decks, so most likely its str and dmg should also be tuned.