Generally, copper is a superior material for piping in a house. Copper has a great track record for durability, but it got expensive enough to encourage a plastic replacement. The plastic materials are a bit easier to work with (no welding and more flexible around turns.)
Copper replaced brass as the best material to use in a house. Brass is good stuff, but brass in house plumbing has been there since around the 1920s. At almost 100 years old, it is brittle and prone to cracking.
So, why would a home inspector take exception to copper used on a steam boiler? Because copper does not carry steam heat as efficiently as cast iron. Cast iron is the best material for that job.
So, why would a worker install an expensive material like copper instead of cast iron? Because copper is lighter and easier to work with.