It's not that black and white. While stealth is something that is either on or off. We can control when and how champions use it.
Which "we" are you talking about?
Riot can control when and how a stealth champion is capable of using stealth and so can a player using a stealth champion, that much is true.
But hardly anyone playing against a stealth champion has much power in how or when stealth is used, and playing against it has very lacking means of counterplay in most instances. Amazing foresight with wards or being a tanky mo'fo with Red Trinket is pretty much it, and even then: champions like Rengar and Twitch with gap-closers or range don't really care they're revealed 0.01 seconds before they launch their sneak-attack.
Eve is probably the most problemed champ when it comes to stealth, simply because her Stealth is pretty much always on.
I'm not so sure about that, Eve's close-range reveal mechanic actually forces her to think about being sneaky, and her stealth is more about information denial with warding than it is a supersize buttsex or panic-button escape kind of deal.
There's a lot more control her opponents have over it than a lot of other stealth dynamics, and it generally feels more like stealth.
That said: could be because the rest of Eve's kit is kind of awkward and counter-intuitive, lacking a lot of tools to really synergize with and capitalize on her stealth.
Rengar is probably the seconded most problem champ since his engage granted by his stealth is too reliable. (Don't know about talon, I don't play him really)
I'd probably rate Rengar as more of the biggest problem champion with stealth.
! is neat but comes up to late.
! changing in size and color or heartbeats increasing in volume could actually be a cool dynamic to add to stealth, make Rengar's ult more like Vi's where at least you know it's comming and you can potentially position yourself in a place that screws it up for your opponent.
When Vi's ult is a good example of where to move forward for more counterplay though... X(
Talon's issue with stealth is more of a Playful/Trickster kind of deal at the end of his combo. He jumps in, unloads his combo, then becomes unteragetable against point-click mechanics when he peaces out while denying vision against skill-shots to retaliate.
Paired with his long-removed silence it was pretty oppressive as the only thing that could be done vs. his combo was move around pathetically, as it also slows and silence used to disable item actives and all summoner spells.
ATM it's a little more frustrating and irritating to deal with than it is strong though, as a lot of item actives, summoners, and abilities can be used before his blades return to Talon for the final hit.
Personally, the best way to make use of the stealth mechanic, is in short windowed moments, allowing the assassin a window of opertunity, but to an extent leaving their opponent some ability to predict what they might do.
I don't really agree there.
Short windows like Vayne's Final Hour are cool for 1-2 champions, but that's ultimatly a Playful/Trickster, Alpha-Strike, Sanguine Pool, Riposte, Counterstrike, Wind-Wall, Divine Intervention, Valor, Spell Shield, or such a variant of short-lived defensive tools with 100% block potential against half or more of the damage methods in the game.
In the case of stealth point-click.
There's already so many variants of that mechanic I don't see stealth being limited to such as a great move gameplay-wise.
Aesthetically it's also a pretty awful place to take stealth IMO. Stealth generally promises a fantasy of...well, feeling like you're Snake in Metal Gear Solid. Or at the very least a Thief in Skyrim. Clever, sneaky, methodical, observant, cautious, and skilled at navigating combat.
Aesthetically stealth doesn't promise just...blocking over and over again when everyone is looking at you.
I guess for example Kha'zix ult, only lasts a few seconds at a time, but it gives him the ability to Juke his opponents,
Being able to turn invisible when everyone is watching him is full derp IMO, which is mostly when Kha'Zix uses that to juke opponents.
Sneaking in with a short burst isn't thematically terrible, but peacing out when everyone is well aware of his pressnce is kind of...WTF. Stealth doesn't work that way IRL. If everyone knows you're there, a lot of arrows and bullets will be directed there.
You just got to look for the more interesting uses of stealth mechanics, and find ways to window their power.
Not all stealth is bad with the right mechanics, that's true enough.
But keeping stealth in the same "untargetable vs. point-click" dynamic it is now would require massive overhauls to certain aspects of a lot of champion's kits that use stealth in such a way that feels proper for their kit. If stealth as a mechanic is intended to make a champion feel more like a Ninja and less like a fighter throwing down a wind-wall, the core mechanics and interactions with stealth need to change at a fundamental level.