At least in the USA, prison rape statistics were not pleasant last time I checked. Nor is the rate of encountering or being around violence and/or intimidation. Not all prisons are as bad as others but non-violent offenders do always get options on where to go.
Most individuals put around constant violence or threat of such are permanently changed, an unpleasant/undesired change. So for may non-violent crimes many would consider prison a cruel punishment. At least with how US prisons are set up.
My understanding is that historically the "unusual" in "cruel and unusual" indicates unusual from everyday reasonable life. Constantly being around violence to an extent it changes the average individual permanently is unusual for every day reasonable life in the USA at least.
(1) willfully causing pain or suffering to others, or
feeling no concern about it;
(2) causing pain or suffering
Intentionally causing bankruptcy for someone (or freezing of their assets) while also threatening them with jail times wildly disproportional to the crime and damages caused by the crime in order to extort a guilty plea before you prove them guilty in a court of law is cruel. Doing so is the willful causing of mental pain or suffering of another human being. If you've ever known another person in the U.S. that has been wrongly convicted of a crime, you'd think it was a cruel system as well. It is especially cruel to those least capable of handling emotional pain.