Human certification has little to do with a concept and more to do with the Factor of Safety in mission-critical design aspects. SRBs are very reliable and predictable in ignition and burn (if you look at their history, more so than very-complicated liquid-fueled engines), but you have seen some GEM boosters fail in the past because their factors of safety for the casting were so close to 1. When your FOS is close to 1, there is little margin for flight excursions such as wind shear, or manufacturing excursions such as air bubbles in the epoxy casing.
Human rating also considers backups for mission-critical components as well. The SSMEs had redundant flight computers, so that in the event one fails there are backups to carry on without mission interruption (which has happened before, on STS-93, when two SSME computers failed due to an electrical short and their backups prevented loss of the vehicle and crew). There may also be mechanical backups, such as redundant ignition pyrotechnics for engines.
NASA's declared requirements though aren't a straight FOS number -- it's a probability of failure number. This is determined differently for different technologies though, but almost always through a combination of simulation and ground testing.
Also of note: crew escape system is an important component of human-rating a "stack". But be aware, you can human-rate a subsystem (such as a liquid-fueled engine or solid rocket motor), but that does not human-rate the entire rocket stack and crew vehicle.
