Hallo,

solid state relais are easy to control. Switching a load at zero crossing is possible. But even up to today solid state relais are sensitive to very high slew rates of voltage. Assume you want to use a solid state relais at mains. Then, if there are certain voltage spikes from lightning or switch noise solid state relais can behave as if a switch-on command at control input has come. So, some filtering might be neccessary or helpful. At least some tens of nanofarad accross switching contacts are good help.
Relais on the other hand are very robust abd not sensitive to fast voltage changes. But contacts suffer from spark eroding. Also, a minimum load is needed for guaranteed contact. Gold layers minimize this minimum load, but spark eroding is increased. So, for every application there is a certain optimum contact material. For very small loads (dry switching) massive gold contacts are optimum, for secure mains switching silver/nickel or even silver/cadmium-oxid contacts are optimum. Never use wrong contact material in your application! This could end up with desaster...

Bye,
Kai