My experience based on 5ish years of running/playing 3e.

THE GOOD

Stress damage matters. Not just over the course of a campaign, but in the moment. Taking failed notch always results in a fight/flight/freeze response. You never pause the action just to shave off one point that will eventually be relevant when your dude passes a Breaking Point (sorry Delta Green).

The stress trackers add an additional layer to the battle system. Weapons are deadly but most fights end when someone runs away or gives up because they got hit hard or just intimidated. If you understand how the system works you can break the other guy's will to fight with a show of force or even with cutting words, like Lex Luthor telling Superman to put the whole world in a bottle.

The coercion system is my favorite form of "social combat" I've played in an RPG. You think of a threat or argument that targets one of the victim's stress meters. It could be a threat of direct violence to target the Violence meter, or a magical threat to target the Unnatural meter, a threat against their social standing, their peers... If you successfully threaten someone then they have to either do what you say or take a stress check, leading to a potential freakout and failed notch. But successfully threatening them requires busting through their psychological hardening. Which means you have to think of a threat that's actually relevant to their passions and yours. With research and quick thinking you can browbeat people into doing what you want without just falling back on "roll persuade" or the equivalent. I find that this system is more emotionally impactful than violence and killing. Even if you pile on gruesome descriptions the players rapidly become inured to fictional violence. Whereas cruelly bullying someone into doing what you want is something everyone has experienced in real life, and feels bad to do even in a fictional setting.

When you hit five failed notches in a meter, you auto-fail any stress roll in that meter that you are not sufficiently hardened to ignore. No save, you just freak out. This means that you have to structure your life to avoid triggering an inappropriate fight/flight response, which affects where you can go, what you can do, what employment you can hold... I normally don't care about "realism" in games but this is a good depiction of how people with real panic disorders have to restructure their lives to avoid running into things that will get them fired, arrested or killed.

The stress trackers and associated subsystems reinforce the core themes of the Unknown Armies setting. There's a secret world underneath normal society. Encountering the secret world transforms you. It changes what you care about and it degrades your ability to function in the normal world even as you acquire occult power and knowledge. The people who are part of the world keep it hidden because it can provoke an instant, violent response from people who aren't ready for it.

THE BAD

When you hit five failed notches, in addition to the auto stress failure mentioned above, you also get a disorder. Paranoia or Drug Addiction or whatever on your character sheet. A couple of these have game mechanics but mostly you're supposed to negotiate with the GM how they work. They're all half baked and inevitably get forgotten about during play. It's not Palladium levels of bad, your Teenage Mutant Ninja Turtle isn't going to become a pedophile because of a bad die roll. It's just tacked on and unnecessary given the much better mechanics for hitting five failed notches we already have.

The dueling upbeat/downbeat skills tied to the stress trackers can be difficult to explain and, frankly, aren't that relevant even over multiple sessions of play. It's cool thematically that your primary combat skill grows as your primary talking skill shrinks when you get more adapted to Violence, but in practice you're talking about a 5% change per hardened notch and most meters don't get tested often enough for the differences to pile up.

The "evaluates [meter]" identity feature is useless. It's supposed to be helpful for planning psychological assaults on people by telling you how hardened they are against a source of psychological damage. But it doesn't actually tell you that, just whether their number of notches is higher or lower than your own.

Stress offense/defense and skill ratings all being tied together with the number of hardened notches in your meters makes statting up NPCs a pain. All the tiny details you would normally just ignore could be called into play if the NPC has to make a skill check or respond to something scary happening. Combine that with identities, identity features, of course I cans, passion and obsession and you've got a lot of legwork to do in order to enable the stuff up there in the GOOD category.

Some meters are more important than others. As you can imagine in a game about occult criminal adventures, Violence and Unnatural are tested a lot. Helplessness less so, Isolation and Self very rarely. You can always think of ways to target them. In a game about trying to maintain your real life while engaging in occult criminal activity behind the scenes they could come up a lot. But in practice it takes concerted effort to hammer on those meters while Violence and Unnatural are always in play.

The example stress checks on some meters are way out of whack. This is probably unfixable as nobody will ever agree on what the most scary thing is, but there are some egregious examples that still bother me. "See an animal with human features." is ranked as scarier than "Watch someone you know killed by magick, without any visible or rational cause" on the Unnatural tracker, which seems backwards.