All steam-heating systems are open to the atmosphere, and that means the pipes and radiators are going to rust.
All that rust works its way down into the pipes below the boiler’s waterline and when enough gathers, problems start. The boiler will go off on low-water because the condensate can’t return quickly enough. If there’s an automatic feeder, it will probably flood the boiler. If the condensate backs into the steam mains because it can’t get through the clogged wet return, vicious water hammer will show up near the ends of the steam mains.