There is rust , and then there is surface rust which stays on the surface and protects the steel underneath. The first type goes deeper and forms corrosion. The type of rust can be an indicator of the alloy mix in the steel on which it forms . In the case of a hub carrier/ stub axle you would want it to be tough and resistant to shock and would expect that the alloy ( steel/carbon ) has additives such as Molybdenum, Nickel, Chromium to achieve the desired specifications for its purpose. These also introduce a good degree of corrosion resistance. Corrosive rust on the shaft if left untreated could mean it all needs replacing some time in the future as it will eat away at the threads meaning larger repair bills. My 19 year Peugeot only had small amounts of surface rust on the corresponding parts and that was with running it without wheel trims for its last 5 years. The 6 month old (from build date ) corrosion on the MG was much worse and seemed to my eye the deep seated type.
I was told many moons ago in my first metallurgy lesson that British Leyland used a steel containing 4% Aluminium for their body panels , this made it more ductile when pressing out the panels and so the dyes in the presses lasted longer. Unfortunately the Aluminium sat at the grain boundaries in the steel and the tiny amounts of electrolytic ( dissimilar metals ) corrosion produced made them rot in no time. Up to now the MG has been great but it is becoming easier to spot where cost cutting measures have been employed.