I can reliably stealth POIs of Tiers 0-4. At Tier 5, it depends on the POI.
Most of those Tier 5's, these days, will include some encounters that cannot be evaded and that likely are overwhelming. There's a limit to how many of these encounters feel like stealth play. If every encounter is a triggers and scripted event, then the stealth player's Tier 5 technique becomes run away, fight on the run, restealth and then come back. There are side-effects to this that can make it very difficult to complete the quest.
Those side-effects include lost zombies that have lost track of the player and wandered off. And, those zombies may not show up as a little red dot depending on the mission and that red dots sometimes seem to disappear. Then min-scripts start to feel wonky as you have cleared an area, spent 20 minutes hunting down a lost zombie, and then suddenly there's another wave to flee, restealth, and then hunt down.
The player is never out of options, which can be its own fun. Modifying the battlefield is one technique. If you know where a minscript is placing zombies and have a zombie to hunt down keeping the minscript from spawning in a new wave, you can mess with their placement.
But, the use of triggers can take away a stealth player's options for trying out-thinking the POI. That is, if you're working hard to get line of sight on concealed positions where zombies might hide, there aren't any zombies there until you enter. Because players can modify the battlefield, including knocking holes in walls, even a zombie encounter in a sealed room can't assume the player has never looked into the room.
At all levels of POI, and even without triggers and minscripts, any vertical entry into a normal zombie volume also takes something away from stealth. This is because zombie volumes (for very good performance reasons) don't pre-place zombies based on vertical distance to the player. This means a player can look down on a room, see no zombies, drop in, and then suddenly be surrounded. Sure, that's a viable encounter and danger is cool, but mechanically this is every vertical entry. Thus, stealth players learn (or should learn) that climbing into, or dropping into, any location is a setup. It is literally a good stealth technique to build stairs to avoid climbing ladders because for some reason climbing a ladder cannot be done using stealth.
So, while I still find stealth fun to play, it is an area that could be improved and I think its really easy for players to conclude stealth doesn't work.
