SSRs produce less electromagnetic interference than mechanical relays during operation. This is mostly due to the absence of a phenomenon called contact arcing only present in mechanical relays, where the physical contacts of the relay tend to spark internally while switching. The reduced interference can also be attributed to the fact that SSRs do not use electromagnets to switch.
The switch contacts of a mechanical relay will eventually wear down from arcing. An SSR will have a longer life because its internals are purely digital. Properly used, they will last for millions of cycles.
SSRs turn on and off faster than mechanical relays (≈1ms compared to ≈10ms).
SSRs are less susceptible to physical vibrations than mechanical relays.
Since the switch inside an SSR isn’t a mechanical switch, it does not suffer from contact bounce and operates silently.