There is only so much maintenance that can be provided to old tiled roofs before the tiles need to be replaced. Unlike terracotta roof tiles, exposed concrete always breaks down. How much so depends on the original quality of materials used to create the concrete tiles, age, historical maintenance (protective coatings) and exposure to the elements. In some cases a tile to Colorbond Conversions is the most practicable long term option.
Post WWII there were a shortage of raw materials to make concrete roofing tiles properly. This led to poor standards of roofing tiles being produced as the ingredients being used was basically all we had on hand. During this time a common mould for roofing tiles was created called a ‘standard’. These tiles were very common on buildings circa 1940 onward to about 1960. Now days these tiles are so pitted and brittle the roofs are generally beyond maintenance, from a practical sense. It is also rather difficult to walk on these roofs without breaking every tile you stand on.
Another concern with roof tiles on old homes is the “wave effect”. This is the effect of having many many tones of concrete tiles sitting on the roof for 70 years or more. Even though most roofing timbers were hardwood, nothing can stop the house moving or sinking with 10 tones or more over many decades. The end result is that the roof looks somewhat wavy with peaks and troughs (and nothing is square)