Spell Matrices weren't perfect. Any magic use opens you up to a horror mark form the nastier and subtler ones, such as Taint.
The big problem with fighting the Horrors is metaplaner shortcuts. It is a trick that any being which originates on the metaplanes can use when on our world, and a very useful one. In order to defend against Horror incursion you need 4-dimensional wards such as those created by the Rites of Passage and Protection. Without warding against metaplaner incursion they can pop in at any time. The problem is that, the way wards work, you'd have to have something like a Kaer. You can't just ward the entire planet.
Now, within Arcologies that have been warded by the Rites of Passage and Protection you are pretty much safe. The chances of a single horror breaking though are minimal. But, and this is the big problem, in order to attack the horrors you'll have to go outside. In order to go outside you have to open the door. If you open the door then things can get in. Kaers were completely sealed for a reason.
The second problem is probably worse, sheer numbers. No one knows how mnay Horrors ravage the land during the peak of the Scourge, save for Chantrel, who lived through it due to the protection of her Eternal Friend, and the Horrors themselves. However, it isn't unreasonable to assume billions of Horrors cross over at this time, perhaps even trillions and you'll probably get into the quadrillions and quintillions if you start counting the individual Dread Iotas.
When your 10-magician horror hunting party gets ambushed by 100s Gnashers who took advantage of the metaplanes to surround you completely, you've got a problem.
The Named Horrors have never been the problem. The biggest baddest Horrors are subtle, take very few victims, and work alone. A small party of adepts decently skilled adepts can take down a solitary Named Horror. This is one of the reasons why the Named rarely stay and fight once exposed. The real problem is the Unnamed.
But this isn't because they'll just kill a few people in the barrens and avoid notice. No, it is the Named that will avoid notice. The unnamed will instead overwhelm with sheer numbers.
Just think about it.
Aazhvat drives individuals insane with pseudo-illusions, one at a time. It isn't into the giant acts of violence.
Artificer builds trap mazes. That'll destroy a city
Bonecrown is a consumate politian who cultivates the love of the masses and uses their loyalty to fuel his power Heck, he's probably Angela Colloton.
Chantrel's Eternal Friend picks one person to grant immortality and kills all of that individual's friends and family. Its a nice plot for a horror movie but not exactly earthshattering.
Despairthought convinces neglectful parents to sacrifice their children.
Druistdadt targets entire riverboats full of Tskrang sailors. Yeah, Tskrang riverboats are very important to the Sixth World's economy.
Fla Tra Lys gives undead servants to whoever plays its pipes.

If you like good music then he's evil incarnate. Otherwise, it ain't really dangerous.
Giftbringer sells crap. Selling crap is the only thing that he does. This does lead to the destruction of entire communities, but only because he is a blatent rip-off of Leland Gaunt from Needful Things.
Hate sows distrust amongst groups by attacking some and leaving others unharmed. Dangerous, but not major at all.
The Hunter of Hunters eats other horrors.
Joie turns people into masochists, causing them to revel in their own suffering as if it were the greatest of joys. There are enough S&M clubs in the sixth world that this is unlikely to turn heads.
Nebis gets killed alot. That's what he does. He gets himself killed. He marks whoever kills them and manipulates them into doing random violent crap until he can steal their body parts. This isn't a very big deal on a global scale.
Nemesis is a goddamn capitalist. Like Giftbringer, he buys and sells; but his goods don't inspire havoc and they're actually worth more than what he sells them for. As a political lobbyist, he's primarily a "tough on crime" guy who believes in high taxes and metahuman slavery. He'd right at home in the Sixth World if he'd rethink the tax issue. If he doesn't already own a megacorp he probably will in the future.
Ristul can't be killed any more than you can kill all of the water in the world. He doesn't exactly act much, either. The Rift is a big exception to this, but he seems content to spread himself slowly.
Taint corrupts magicians. If you aren't a magician then it is not a big deal.
Tempter corrupts well-meaning adventurers by offering them the power to do good. If you aren't a well-meaning adventurer then it isn't a big deal.
Ubyr, okay, Ubyr essences drains entire cities at once. He's a giant destructive essence leach. I'll give you that one.
Verjigorm hunts dragons. Terrible, if you are a dragon.
Ysrthgranthe abuses Aina. Terrible, if you are Aina.
But, just look at the Unamed
Baggi eat people. That's all they do. They eat people whole.
Crystaline entities Possess homes, garages, and toolsheds and then makes people inside grow spikes if it doesn't kill them outright with spells.
Dread Iotas are bacterium or viruses. But worse, each and every one of them is sentient and capible of using magic. They exist to infect as many people as is possible and they will communicate with their victims and manipulate them in order to propagate the infection.
Gnashers eat and jump. Those are the only two things they do. They don't even poop. They are truly omnivorous and their appetites are insatiable. They eat anything and everything untill they were full, which is never. Enough Gnashers could eat the entire planet.
Kreescra Give people nightmares and prevent the healing of wounds.
Mindslugs crawl into people's skulls while they sleep and feed of of their brains.
Slipshades are harmless unless you trespass onto their territory.
Wingflayers are flying clouds of razor-sharp splinters. They shred any living being in their path.
Wormskulls, I'm not sure what they do.
The unmaned are generally all much more obvious and, more impotantly, they cause more destruction due to their sheer numbers. Gnashers can eat entire cities and without 4d wards there is no way to prevent them from infiltrating a city. Likewise, there is no way to prevent a Crystaline entiry from possessing your home and killing you in the most painful way possible without a 4d ward. Dread Iotas can potentially infect and kill millions, if not billions. They'll make VITAS look like a freaking rhinovirus.