I go to @
gugie for all my brazing needs. He's a mechanical engineer, so he'll go into the technical details and explain to me why whatever I have in mind will or will not work if I ask, but he's also a very practical guy so he'll say "What could possibly go wrong?" or "Don't do that" if I don't ask for more info.
I'm not a frame builder or mechanical engineer, but I do have a degree in physics, so I'll offer some completely unqualified speculation. Assuming a quality job in the braze on, I think the strength depends on a few things -- the material used for the brazing, the shape and size of the contact patch for the brazing, and the geometry of the force applied (angles, lever length, etc.). If you tell the builder you want to use a rack with N pounds of cargo, they ought to be able to produce a braze on that will hold it. If you say you want eyelets for fenders and then after they're done you ask if it will hold a rack too, you might get a vague answer. Typically these things are way stronger than they need to be, but if you didn't ask ahead of time you never know.