The Council has been on the backfoot fighting them all over Europe, to the point that they have effectively abandoned NA.
As we see in GS, the White Court is coordinating with the Paranet and engaging them all across the USA.
The Formor have been (without provocation) attacking everyone on a Global scale, for the last... what two+ years? It gotten so bad that the Masquerade is falling apart despite the superhuman levels of head-in-sand that Humanity has in the DF.
So yeah, somehow they're the biggest threat in the series, despite living in a vacuum.
Heck, a couple of their Redshirts Turtlenecks are dangerous enough that Harry needed to be rescued from them, whereas the Red Courts top assassin went screaming like a little girl at the sight of him- and that was before he became the Winter Knight.
I mean, they're not just a rag-tag group. They're a Nation. A Nation that is shitting all over everyone, and yet nobody has punched their ticket yet. It's as if... I dunno... Sokovia decided to start attacking every other nation in the UN, but nobody did anything about it because they're "just going after soft Targets"
I kind of agree with you there on some points, but the Fomor don't seem to be engaging in full on fighting with them. They're working on grabbing werewolves off the street, similar to the plot in White Night, and adding those little powers to their own.
Seems to me that they're staying mobile. In "Aftermath," they're in town for, what, two, three days? They were closing up shop and getting ready to vamoose.
Winter has been on the backfoot, so to speak, since Mab was spending most of her time keeping Harry alive, then training him to murder her daughter, then keeping watch over Molly while getting back at Nicodemus—and acquiring some apparently major artifacts for the coming fight. I expect that to change when the Peace Talks fall apart; Winter is coming back in a big way.
It's not surprising that the White Council pretty much bounced out of North America; they never really cared about it before, and only had a handful of wardens keeping an eye on it, despite having the same population as half of Europe. They don't seem to be particularly threatening to wizards in general; Ramirez was injured, as I recall, but the circumstances of that are unknown. Plus, he kind of gets injured a lot (I think he's ended every book appearance in the hospital, including Changes—he was down with whatever biological weapon was used against the Council). He's like the warden version of Jeremy Shockey.
The White Court... I don't know what it is they're doing, so I can't really comment on it. I'd assume that they're gathering intelligence with the Paranet to protect their herd. And I further assume that their attempt at resolving the situation is going to be seen in Peace Talks.
In fact, I don't think I've
ever seen the White Court Vamps do anything useful in the war, except help Harry, so I don't know what it is they're doing.
Anyway, I think the Fomor are a threat because they
did come out of nowhere, and nobody was prepared for the things that they can do. The White Council has to shift gears to fight a totally different kind of conflict, with a target from the
sea rather than Central and South America. They likely didn't even take them seriously at first, figuring they were just some upstart nothing organization. I think they've had to reconsider the threat level they represent, even though I'm positive Cristos is busy distracting them by inciting unrest.
All that aside, I do agree with you that I don't generally like the idea of introducing a new threat totally out of left field and selling them as bigger or more dangerous than anything else. It's what Supernatural (a guilty pleasure of mine) did with the Leviathans a few years ago; even though they weren't capable of anything worse than any other monster, they were sold as more dangerous than
angels, despite us having watched angels travel through freaking time on more than one occasion. They spent way too much time telling us how incredibly deadly and scary we were supposed to find them, but at most they were comically violent. This was instead of showing us what they were really capable of.
Now, I trust Jim; he wouldn't do this without a reason, and I'm convinced that the Fomor have been in the plan for a while; on the Wiki, it speculates that Nicodemus's "beasts" were provided by them, for example. We haven't really seen the Fomor interact with any of the major players on the page; I think that, once we do, my opinion of them as a threat will change.
That aside, I did find the one in Aftermath threatening (Nobody? That was his name, right?) I could see an army of him giving anyone pause.
DISCLAIMER: I haven't read Even Hand yet, so my only direct knowledge of them is Aftermath, Molly's scuffle in Ghost Story, and the Octokongs.