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

I'd vote that it makes more sense to put a delay on for rangers. I'd never really understood that about ambush. I just don't see why you should be prevented from attempting to backstab someone if you aren't hidden.

Because ambush is NOT backstab. It's designed for the different game mechanics for the different classes involved. Otherwise, we might as well do away with them both altogether and do something stupid and generic like 3rd Ed D&D did when they made "sneak attack". Sorry but that took the fun out of it for me.

In fact, ambush was designed for rogues, not rangers, and was only added to rangers later during the endless tampering with "class balance", which we all know now is a complete waste of time to try to perfect.

Rogues got ambush so they would not be assassins that dual wielded and riposted.

Game mechanics.