I've had a few weekends to play A21 and a number of the new POIs through Tier 4. I had a pretty good time in the OPS Distribution Center. (I think that's the name.) Before that was a "Hogzilla" POI. I've been "double dipping" them so the first time I was more likely to fall for tricks and the second time I was somewhat informed, though sleepers don't always spawn in the same locations, so it can be still a bit of a mystery.
In A20 I'd say 80% of the zombie volumes could be handled via stealth and of those that couldn't, the encounter was usually mild enough that switching from bow to club was more than enough to cope. The revised Crack-A-Book (Tier 5) could be cleared 100% with stealth.
My second pass through OPS Dist Center (Tier 4) today was pretty satisfying with stealth, but the first pass had some surprise moments. I can see why folks aren't happy. Again, most of the surprise activations weren't a major complication, but the final encounter got me into a running gun battle and a significant injury. I can get too confident and that can set me up. It did in this case. You can't always just "crouch and go." Sometimes you have to control your movements to keep the stealth meter low. In the second pass through I cleared the final encounter entirely with stealth and without destroying part (walls) of the environment to give me shots. This was AGL 5, and 3 Perks towards sneaking and stealth damage, all cloth armor, primitive bow 6 with 4 mods. I even took a screenshot while standing amongst the final loot with bow aimed toward the last zombie. I was super cautious in that approach, using single steps to keep the stealth meter around 1, rather than an extended "crouch and go" stealth walk with a stealth meter around 8. I kept my distance, clearing out boxes to give me good angles to shoot.
Hogzilla has a couple of spots I don't know if I can stealth. There's a pair of zombies that have a "rear slope" position up in the ceiling and they activate when you enter the room. I think there's 1-2 more Zeds in that room that activate. Here I feel a little cheated, but it isn't a huge deal as I have a way to withdraw and trade space for damage.
I've heard other POI designers talk about the clever things they can do with new volume options. I've had my concerns that making zombies appear behind people in areas previously cleared are likely to be unpopular. In some cases, designers are trying to suggest zombies wandered in behind the player. I see some merit in that. The thing is players aren't going to get a chance to see the designer placed the zombies by the doors, by the air vents, or wherever they were supposed to come from. Players are just going to know there's a big ruckus behind them. Plus, players are used to zombies stumbling around outside the POI. We can hear them. They're kind of fun when they detect you and "add" themselves to the situation because they'll do neat things like tear a hole in wall for you. I mean, I like exits from buildings and I'm happy to loot and look around why a zombie makes one.
I guess I'm rambling here. The ideal stealth experience is a matter of individual taste. Designers have some new tools. I think with details and feedback, a happy medium is possible, but I think A21's new tools can be taken too far. I don't think a POI has to be 100% doable with stealth. I do appreciate the POIs that make me think and take my time, presenting a stealth puzzle. It isn't the most efficient way to level, but I find it fun.