There are a few problems with replacing electromechanical relays with solid state relays.
1) Most SSR's cannot control an AC signal. They are only rated for DC. They will usually conduct all the time with a reverse biased drain-source current, due to the body diode of the MOSFETs.
2) SSR's can be damaged by exceeding the maximum current through the photodiode, which is usually no more than 10-20mA. A relay will take 100's of mA through its coil. So they aren't a drop in replacement.
3) They are faster than electromechanical relays, which can cause problems for some circuits (the circuit may be designed to reset in the time the relay is transitioning between the two states, for example.)
4) They are sensitive to ESD, electromechanical relays are not.
5) And finally, most importantly, they do not click when they change, which makes them kind of boring. I love hearing my scopes' relays click when you change the attenuation. Don't underestimate how awesome this is.