I am assuming that the method for measuring the water content of a hydrate you are referring to is heating it up and measuring the difference in weight. The reason that this is not suitable for measuring the water content of all hydrates is that not all chemicals will survive the hot temperatures required to remove the water. Some chemicals will decompose at high temperatures, thus rendering the measurement useless. Another reason this may not be useful is because some hydrates are either very unstable or reactive in their anhydrous form (without water). These chemicals can then react or decompose without the water present to coordinate with.