(5.5E) Is the 9th level Rogue Trickster ability Magical Ambush severely limited due to ONE word used in description?

Edition - 5.5. Magical Ambush is a 9th level Arcane Trickster ability. It reads "If you have the Invisible condition when you cast a spell on a creature, it has Disadvantage on any saving throw it makes against the spell on the same turn.".  I am assuming that invisible does NOT equal hidden. A Rogue can use a bonus action to get hidden often. Invisibility obviously requires a ring or spell, etc. And if you use it that way, it just seems to be a waste of resources.

I am going to make the case to my (fairly reasonable) GM that this 9th level ability is severely limited and could we consider allowing it to be "hidden" instead of "Invisible".

How would you rule? What am I missing?

The hide rules are some of the more hotly contested ones in the edition. I expect lots of people will chime in on this one. I’ll try to cover it as neutrally as I can.

If you are obscured, or behind cover and make a dc 15 stealth check, you gain the invisible condition; the hide action makes you invisible. So far, it’s pretty easy to say the power would work well. BA hide, hit your DC, action, cast your spell and the target has disadvantage. 

There is, however, a lot of debate about it. Basically, if you go around a corner and hide, as a result you have the invisible condition. What happens when you step out from behind the corner?

Theres a bit that says using the search action is the only way to find an invisible creature, because that’s what the rule says. Since you are invisible, you could walk right up in front of the guy, do a little dance, moon him, whatever, and he can’t see you unless he searches. 
Others say, that when you break cover, the target would certainly see you, because, well, you might have the invisible condition, but you’re not actually invisible, per se. You just hid really well, and once you start walking around, you’re not hiding anymore. 

So, it will be up to your DM to decide how to run it at your table.

They somehow made the 5.5 hiding rules even more complicated than they were previously.

To be fair the rule does say: "On a successful check, you have the Invisible condition while hidden." Implying that the "hidden" part can be broken (presumably when you are in line of sight of an enemy) and would therefore lose the Invisible condition. But it takes quite a bit of obfuscated logic to get there.

I think using the word "Invisible" as the condition is the real problem. Its confusing: The Invisibility spell makes you invisible, but so does ducking behind the wall! These are obviously two completely different ways of "not being seen" - in one you can stand out in front of a guard dancing without being noticed (if you dance quietly) but with the other, realistically, you can be seen if the guard happens to wander around the wall.

Personal opinion: They should have made "Hidden" the actual condition.

• If you succeed on a Hide check to duck behind cover you become "Hidden" until you end your turn in line of sight of an adversary or an adversary moves into line of sight of you.
• The Invisibility spell causes you to become "Hidden" until the spell ends, if you attack someone, deal damage or cast a spell.

 [Or something like that]

Hide definitely applies invisible, the question is whether it does anything else (such as making people unaware of your position) and how easily that invisible condition is lost. However, that dispute isn't really relevant to the OPs question -- if you could make an attack with advantage due to being hidden, you can also cause victims to have disadvantage against a spell you cast.

Thank you everyone. I don't know how I missed that. :-)

It is strange, yes? Its a rogue feature and... it was my bad for not catching that. Nothing like a forum where crazy people show up and stir it and then realize they totally read it wrong.