The big downsides with horrible prison are that criminals will have a huge incentive to do anything to not get caught and convicted, including violently resisting arrest, shootouts, targeting witnesses and so on. It is mostly effective as a deterrent against financial/winning-relates crimes, but mostly ineffective against most other types of crimes. Plus, horrible prison is much less likely to actually reform the person in question.

Statistically, the Norwegian system with "holiday prisons" works well, with very low recidivism rates for most types of crime.