Hmm point, although in terms of making no mistakes I think that they possibly had a situation where they were primarily focused on defensive tactics and as such neglected their external information gathering attempts that would have allowed them to be more aware. Like for the We situation they would write them off as giant spiders and not bother to investigate further, which if they succeeded at would have tipped them off to the Dwarves being on Friendly relations with them...which would throw the players for quite a loop.
I think that the problem would be that of motivation and resource expenditure.
In order to investigate the spiders sufficiently to determine the spider-Dwarf friendliness, they would probably have to send some scouts all the way up to where the Dwarf sections of the Under-Citadel start. There, they could see that the spiders were
behind some Dwarf fortifications.
But in order to get scouts and spies that far, they would have to go through the entirety of spider territory. There is no motivation forcing them to do so; they don't
know that they have to spend their scouts' lives so much and send them all the way there, in order to get this information.
They have to judge the situation by looking at, and working with, what they have. And the appearance of the situation is giant spiders.
Maybe if they'd done more really deep scouting probes? They might have learned more of the Dwarfs.
But their only outpost was Und-Uzgar (I think it was said somewhere that that was a Clan Mors post, right?), and that got wrecked early and quickly and thoroughly... In fact in hindsight, I'm not sure how they were able to send Skaven out to Und-Uzgar? Was it an isolated branch? Did they manage to send Skaven through the Citadel, before the spiders moved in? Or through the Underway to Kvinn-Wyr or Mhonar or something?
Mhonar probably makes sense; they probably exited through that. But, it was probably a "... And then your whole party got eaten" risk for them each time they did so. A Scylla and Kharybdis situation. 'Get somebody topside, but pay for it each time.'