There's no question that there is a lack of information about the Watch, and much of it is contradictory. But one way of assessing it is by its results.
The fact is, the Clans (even the Adders and Sharks, honestly) are never portrayed as being particularly savvy about what is going on in the Inner Sphere. Even the Wolves in Blood of Kerensky, despite having access to the Wolf's Dragoons' reports and a better than average reason to have high confidence in them, seem to have a poor understanding of what is going on in the Inner Sphere and what things look like. They seem to have been taken completely by surprise by even so massive a military build up as Operation Bulldog.
But, Intelser and the Watch never let even a hint of their existence get back to ComStar, who were just as clueless about the Clan's origins as anyone else despite ROM. Between the Watch and a policy of "loose lips sink ships" the Clans (mainly the Falcons) were able to keep repeated attacks into the Inner Sphere a surprise, constantly taking the Lyrans unaware even though the Falcons based on occupied Lyran worlds likely saturated with Lyran agents. The Falcons were also able to keep their military build ups quiet from the Inner Sphere. The Jaguars were less successful, but even they were able to nearly fool Bulldog (which was surely supported by a huge push from the ISF and ComStar ROM) and they were regarded as the least forward thinking of all Clans and were being set up to be whipping boys for the story.
Now, I can't say if the Clan's counterintelligence capabilities are due to the Watch, or just due to their insular nature (if every trustworthy person in your life comes from one of 25 bloodlines, it does make it easy to know who to trust) but it does seem to be a strength of the Clans in general.
They don't seem to be very inward looking, of course, given the growth of the Society, but again that's probably Clan nature, and some of the Falcon books hint that they could have done more had they been set on the problem more aggressively.