Stealth as a WHOLE is actually a healthy mechanic that has counterplay
the problem is that there are specific abusers of stealth that LACK that counterplay
Ex:
has a form of stealth that forces her to either:
A:
Get CC but let enemies know where you are coming from while giving them a huge chance to run away
or
B:
Get some free damage but you can't CC anyone with half a brain
In both situations, Evelynn loses out on something which makes it more fair for people fighting against her Stealth because Evelynn can never have the best of those two situations without giving up the other situation
Evelynn's main goal is to kill valuable targets until EXTREME lategame(50+ minutes) in which she becomes a way to pressure enemy ADCs off from fighting teams and she looks to 1v1 skirmish before teamfights
Second Ex:
Kha'zix
Kha's stealth has NO downsides to using it once it's been activated. The only time that a Kha player can misuse their stealth is when they are entirely alone(and know that they are alone) which is EXTREEMLY unlikely to happen
Kha's stealth enhances his main goal, to kill valuable targets until lategame where he wants to kill targets before teamfights
A Kha player can do more than the Evelynn player with the damage they are both given BECAUSE his stealth allows him to do his job better than what Evelynn can
This in turn means that Kha's stealth is unhealthy due to the interactions it has with his kit. Evelynn's stealth, however, is extremely healthy due to how it forces her and her opponents to play around it(with chances for both sides to mess up horribly).
It is nigh-impossible for a Kha'Zix to mess up spamming all of their abilities on an enemy. It is up to Kha's enemies entirely to mess up in order for Kha to win an even fight(1v1, 2v2, etc)
It's extremely possible for an Evelyn to mess up her abilities if she doesn't use them correctly. It's up to both Evelynn to succeed and up to Enemies to evade Evelynn's advances for situations to play out properly
If an enemy messes up against Kha'Zix they will lose if they are isolated. If an enemy messes up against Evelynn they have a chance to succeed because the Evelynn CAN mess up
In other words Stealth as a whole is fine but champs like Kha'Zix, Akali, Shaco, and Twitch are outliers whom abuse stealth due to their kits not functioning around the sheer POWER of stealth
If each of those champs had their power levels stripped until proper usage of their stealth mechanics made them good or bad(depending on player skill entirely) then that would make them balanced
Unfortunately Riot BARELY knows how to balance their own game, so we are stuck in this nightmare-land until Riot fires current balance team and replaces them with McDonald's employees who play video games(and probably know more about game-balance than current balance team)