Just an issue to consider: when an assassin backstabs something while not hidden, they miss, AND can't attack again for a duration of time, usually ranging between 5-9 sconds.
Rangers on the other hand can't ambush if they are not hidden, they get a message saying "How do you expect to ambush when you aren't hiding?" Because of this, rangers just hide and ambush without having a duration of time where they can't attack.

Would it be possible to make it so the same thing can happen to assassins so they can't backstab unless hidden?

I think it'd be more fun to make rangers have a similar long stun when they ambush without hiding