I’ve done both. The original as written 2x through, and the redux-style earlier this year to see what it was like.
The original one is much harder. It has you doing weighted pull ups, weighted dips, heavy goblet squats, and SLDL’s with shrugs as the assistance. These are supersetted all the way through (even during the warmups), keeping the intensity high throughout.
The redux version has you do the warm ups and 531 sets without supesets, and the supplemental work and assistance are done as circuits (two easier assistance moves between each supplemental barbell lift). Redux gives the option of lower volume in the supplemental work (5x5 FSL, for example compared to BBB in the original).
TL;DR: the original krypteia is very hard and not a long term, sustainable program but more similar to a challenge assuming you are really pushing assistance as prescribed. I recently did Building the Monolith (a 531 challenge) and the original krypteia is similarly challenging. The Redux is something you can run for the long haul, and is essentially a traditional 531 program, 5’s PRO FSL with the assistance/supplement done circuit style.