WD40 isn't bad for getting stuck stuff unstuck, the problem is that it leaves a residue behind as it evaporates and it is more volatile than oils, atf, etc. So, if you allow it to remain in the hub, it's not good. Using it as a flush and then flushing it with atf, or whatever, is ok, I suppose.
Apparently, the primary ingredient in WD40 is a light hydrocarbon closely related to something called Stoddard solvent which is a lot like kerosene. It's pretty much the stuff that's used in solvent cleaning systems. So, yes, like the solvent or kerosene it will dissolve gunk and "varnish" left behind by the evaporation of other oils/lubricants. You can also use ATF cut 50-50 with acetone.