I think there are two possibilities: the brick itself, by some trick of composition and faulty firing, is susceptible to this kind of corrosion, and that something very weird is going on behind the brick.

t=484690Galvanized water supply is something I don't like, because the pipe is corroding from the inside, and eventually the corrosion can lead to blockage of the pipe.Iron waste pipe is somewhat more modern than alternatives like say clay ...

of the tanks were steel, and petroleum products are corrosive.

They may be leaking water actively or show signs of significant corrosion.

I am born and raised in south Florida, University of Miami alum, BS & MS in Biomedical engineering and now working as a Sales Engineer related to coatings and corrosion (exciting topic, I know).  

If your water is not highly corrosive and the plumbing is reasonably accessible, and not leaking, I would leave it alone.

The downstairs unit is currently on oil and the system is currently functioning but the tank has corrosion and the furnace is in rough shape.

I don't know how old your lines are but we discovered corrosion buildup in our 3/8 supply lines within the shutoff valves. 

For example, they might say something like "There is significant corrosion and to repair I might end up damaging other things.

Like anything else they have issues mainly corrosion.