The bark mentions avoiding those of "the order of thieves" which is meant to imply those that are involve in thieves' guilds, along with the deep entrenchment of politics and currency that entails.

There is no opposition to the thief class in and of itself. One could even join the cabal, although obviously joining when you have chosen certain paths would be a dumb idea, both ideologically and practically speaking. (And any thief would find himself restricted to some extent.) Assassins could be in the same category as unguilded thieves of certain paths. When it comes to these two classes, I expect Outlanders to use their judgement when determining whether they are trustworthy companions. Obviously there's a big difference between a woodsy felar trapper who doesn't like the cities, and a slimey pickpocket who is obsessed with the aquisition of more gold.