The fallacy of this statement "the class is fundamentally broken" is based on your perception of what stealth should be. As we have learned playing many different types of stealth games over the years, stealth can be handle many different ways (from Hitman to Dishonored to many others).
If you walked into a POI and every sleeper instantly woke up and targeted you no matter what you do, then yes the stealth in this game is fundamentally broken. However, the developers have gone the route they did where if you spec into Hidden Strike and From the Shadows (and kit your gear correctly) then you will clear out a significant majority of the zombies without them knowing you are there. For all of the zombies that are awaken though, the game has the mechanic where you can strategically go backwards, hide from them, and then re-engage them later and get the Hidden strike bonus.
Any true stealth player doesn't go into the higher level POIs without a silenced pistol or SMG. It's even advisable to have a larger weapon (shotgun for example) if all hell breaks loose and you are getting overwhelmed.
Based on your comment, you don't want to have to retreat. That is your choice. But when you disregard one of the benefits From the Shadows gives you, that doesn't make stealth fundamentally broken. That is a choice that you made that doesn't take advantage of all of that perk's benefits. That is like only eating boiled meat the entire game and then stating the food regeneration system is broken because you are only get 10 food for each item consumed when you have a total of 200 food.
You are right that I have a certain expectation of stealth (not a fallacy tho

).
But it is based upon a simple concept:
stealth is all about control (funny, because its in the AGI tree

)
I control when I start the fight (if I dont mess up). I am the person that has to decide if its too risky or if I can do it.
This control is yanked away, as soon as it is a roll that every zombie gets upon entering.
Compare the two "types" of Z' state:
normal in the wild ("awake"): it is ONLY triggered by noise/sight. If I am careful enough, I can stand right behind the Z' and it will not spot me, granting me the chance to do a stealthkill.
in a POI this is not the case. I can do everything right. No armor, no light, no trash on the floor no meat stew in my pocket (oh wait

)...
and yet the game forced this fight upon me.
In A16 (I don't know when this change occured) if I did everything perfectly, sneak would get me through every POI.
I still had to fight once in a while, because no human is perfect. And with a bit of tweaking, this system worked perfectly.
Now, it doesnt matter what I do... because the decider is still the game rolling dice based upon stats.
And I disagree: if I am confident in my ability, I don't take anything with me in hitman.
There is no reason why I shouldnt just specialize in a perktree that is actually useful, instead of having a half-working perk that does nothing except this (hyperbole again!)
I would love for TFPs to measure stats by percentage:
Tree most skilled in: (I think Fort >50% then STR and then all the others)
So we have:
STR: direct confrontation
FOR: direct confrontation
PER: shooting from afar or blowing stuff up
AGI (based on your playstyle): sneak until seen, then direct confrontation (but more evasive)
INT is the only path that needs actual setup. Place turrets, wake a few up, get the stunbaton...
Why does every Path lead to direct confrontation? I don't want to fight Z's. I have bloodmoon for that.
I want to be a survivalist and not fight unless I have to or use stealth going one by one...
but now even stealth leads to confrontation...

almost as if there is a designdecision behind that that would explain these changes...