Solid State Relay vs. Electromechanical

Really depends on the environment and stress. Vibrations, Intrinsic situation, Weather, Corrosion, Temperature, Character and type of the load. Elaborate your question pls.

There are many conditions to use the SSR. Mainly there are two reasons;

1) When a load switches too frequently, we use SSR to beware of arcing problem.

2) When we want to switch the load for a very short moment (jogging).

Depends on what you mean by "harsh environment".

SSRs are good for long lasting, high speed solutions, but they reject heat that must be dissipated, approx. 1.5W per amp per phase. So if they need to go into a sealed enclosure in a harsh environment, then you must deal with that heat, i.e. extreme over sizing of the enclosure (if there is a low enough ambient temperature to allow convection dissipation) or an air conditioner, but A/Cs do not usually last long in "harsh" environments. You can sometimes use other means of external cooling (vortex cooler etc.), but there are other trade offs, such as a need for instrument air.

If by "harsh environment" you mean explosion proof, then often times the extra enclosure size is so expensive you may have to look for other alternatives.

"Harsh environment" conditions can be abated by using equipment enclosures with the correct Ingress Protection rating, then regular equipment can be installed within.