Why would it probably be very difficult to determine the chemical and physical properties of a newly discovered synthetic element?

With newly discovered materials, nothing is yet know about them. Scientists don't know how to conduct physical/chemical experiments without damaging the element. Lethal substances or corrosive materials can be set off from combustion or decomposition. It would be difficult to determine the properties because no one knows what the element is made of. Chemical properties are often characterized by reactivity with other chemicals, such as toxicity, coordination number, flammability, enthalpy of formation, the heat of combustion, and oxidation states. It is difficult to measure those things when you have no idea what an element may be giving off.