The main drawback of solid fuel is that once it burns, there is no throttling down. It will burn at full blast until the fuel is exhausted. This makes them a liability both for safety reasons as well as useless for any mission profile that is more complicated than 'generate a lot of thrust quickly and in one go'. And even as launch boosters they are not optimal. Ideally larger spacecraft want to throttle down and back up again during the early phases of ascent to be able to safely and efficiently push through the period of maximum atmospheric stress on the vehicle. A purely solid fuel first stage doesn't have that option. As the bottom line concluded, this technology is stricly limited to putting payloads into simple orbits... or ballistic trajectories.