Have you actually driven a vehicle with the Holbrook springs or is it all theory? That conversion does give quite a bit of body roll. Manageable on the street if you’re aware of it but will never be a slot car. In reality it smoothes out washboard roads, flexes quite well, and holds up well. It was designed as a kit that most people could install with careful measuring and basic hand tools without welding. At that it excels. Is it as good as a Wrangler conversion? Debatable. YJ is much more stable but typically doesn’t flex or ride as well off road. The Holbrook springs came with a wedge on the rear springs for pinion angle. The way they were designed there was no more mods needed (other than 4 spring mounts that could be bolted on) than any other 2 1/2-3” spring lift. I’ve driven and wheeled both conversions. I’ve actually installed both conversions. All depends on what you are looking for, what your skill level is, time frame is, etc. As far as how many vehicles have it? Are you talking factory? That would be something called liability. If you’re talking YJ springs, this system was designed before YJ springs were readily available and yet still works well with minimum work comparatively.