There were two wars that devastated the properties of the royal families--WWI and WWII. After WWI, the homes were generally in structureal soundness, but due to massive financial losses, many of the possessions were sold off to get cash.
During WWII, the buildings themselves were often the vicitims of air bombings, but by that time, the original furunishings were already removed or sold. There is a not a large collection of original furnishings still in museums and the palaces. Some exist, but losing two huge wars does not leave much behind.