Is the Arcane Trickster’s Magical Ambush intended to be used with spells that have a V component, or does speaking the mystic words give you away before you can benefit from the feature?The intent is that you benefit from being hidden but then reveal your position.
— Jeremy Crawford (@JeremyECrawford) November 17, 2017
“Huh. That wizard went behind one of those crates. I lost track of them.”
“Allakazam!”
“Oh no! I am so startled by your sudden appearance, that I’m now not as good at resisting the effects of your spell!”
“Wait, really? But it’s a Constitution saving throw.”
“Yes! I am so shocked, that my immune system is less able to resist the poison you conjured! If only I had been able to see it coming!”
“…what? Are you serious? I mean, I could understand if you were making a Dexterity save to dodge a fireball or something, then being surprised might make that harder, but…”
“Nope! All types of saves! Even Charisma! You shocked me so badly, I like you more than I did a moment ago, and am more easily charmed!”
“That’s bizarre and inexplicable. But useful, I guess? I assume it also makes it easier to hit you with spell attacks?”
“No! Absolutely not! Don’t be absurd! What – you think jumping out and surprising me is going to make it harder for me to dodge a Ray of Frost that you sling in my direction from a tricky angle? Hah! You wish!”
“So let me get this straight. If I could cast Heat Metal, which automatically affects the armor you’re wearing and causes it to glow red hot, you’d have a harder time resisting the damage from the heat if I surprised you?”
“Yep!”
“But if I sniped you with a surprise Firebolt from a hiding place, it’s be just as easy to dodge as if I stood right in front of you?”
“Yes, exactly! Now you’ve got it!”
“No one wonder people don’t play Arcane Trickster. This class is sort of all over the place, and kind of a mess.”
Funny as your post is (and it *is* cute), it’s missing out on a key underlying metaphysical aspect involved: it *isn’t* a “wizard” (as you cite) ducking behind some crates… but an Arcane Trickster(tm). The mystical blending of their rogue-ish abilities/cunning and their (limited) development of spell-casting allow their spell casting to in-a-similar-if-different way express their “sneak attack” effect relative to their spell-casting when they cast from a hidden position relative to the target (the difference being that instead of precision-related damage being added, the spell effect is able to subtly/cunningly manifest itself… which is reflected by the target having Disadvantage on the relevant save). That’s not at all so strange an ability for such a rogue-caster to have as part of an archetype feature — it isn’t that the target is “oh my — I am so shocked/surprised/flustered” that the spell is more effective, but rather the Arcane Trickster being able to direct/influence their magic more effectively when in a position of doing so while “hidden” (as defined in the game mechanical terms), again just like with how their SA feature can potentially work in similar situations.