Have you ever noticed, the sound difference when up close to an object vs max distance? Like chopping a stump or a tree up close vs paces away.
It's hard to quantify our experiences since we don't see the game's variables or code.
What I would say is I
believe being distant from things you're interacting with seems to help. That is, I
think your manipulating and destroying things does affect your stealth meter (as you can visually see) but that I think zombies perceive you (and that stealth meter) based on your location. Being farther from a zombie helps.
Chopping a stump is an interesting choice and that would be outside where things are really easy. That is, I can kill the zombie from a long ways away and then chop the stump.
Harder would be taking down a locked door in a POI. You don't know if there's a zombie 1 block away on the other side, or not. But if I can be three meters back from the door will the zombie be given the chance to perceive the door next to the being quietly destroyed or will they be given the chance to perceive me now 4 meters away, possibly muffled by the door/wall between us?
The big weaknesses are outdoor areas during the day, some trigger zones, and a zombie that might be too close to a doorway/too close to me and another zed, and I have to choose which one to assassinate and the other to take toe to toe. From my experience in the past, T5 POI's in almost their entirety are impossible to stealth, just outright. Virtually every single room is a trigger area in some way, shape, or form. You enter one room, 20 zombies spawn in and fall from the ceiling.
I can't reconcile this with my own experiences. Taking it bit by bit...
Outside is really easy for stealth kills because you can have really long ranges. But you don't specify the weapon of choice here, so maybe you're talking about using a knife? If so, that squares with what I've seen. A knife is really hard to use for stealth kills, mostly because it is hard to avoid the zombie turning around to face you. (I need to use more rocks and see how that goes.)
Killing single sleepers in a group doesn't feel that hard to me, but it may be a matter of expectations and differences in level, perks, and gear. If I'm low level, without perks, and in the starter gear, then I could seeing killing one, waking the other. Or in a Tier 5 with special volumes and scripts, there are some pretty unique situations. But between those ends, I think stealth rocks.
I'm still just getting into the Tier 5 POIs for 2.6 and I can empathize, but I also think you're exaggerating. I did the 2.6 Crack-A-Book and the entire middle of that POI (other than the basement and roof) were a lovely stealth experience. Even with those two floors being wild, stealth wasn't useless, but it certainly involved some rough spots, some resorting to using the AK, and disengagement to regain stealth. A solid diet of those nasty encounters would certainly get disappointing. (I captured that 2.6 Crack-A-Book on video, though haven't shared it. I should figure out a way to do that.)
All of the Tier 4s I played (and lower) were great for stealth.