Shields & Armors lack standardization. IMO it' one of the projects/issues the devs should resolve sooner rather than later, cause the work compounds with each release.
Basically almost all modern (2015+) items are all over the place in terms of:
- Shield damage dice & weightedness
- Shield AC
- Shield DR
- Shield crit profiles
- Armor AC
- Armor MDB
Please standardize the curve for all these aspects, and then place all shields/armors on it (with intentional exceptions wherever you see fit obviously).