I have the Arcatech GP ballhead. From what I understand, out of the original first batch - since it's going on 14 years old now. I would also think that it's pretty similar (if not the same) to the Ultimate. For plates, I currently have...
- Arcatech plate for the K100D (this came with the ballhead)
- Krik L-Bracket for the K5
- RRS L-Bracket for the K1
- Sunwayfoto flat generic plate PU50 that I have on the bottom of my Nodal Ninja 3
- HejnarPhoto 14mm rail with a Y that is also AS compatible - support for my A-300mm/f4 lens
You have gotten my attention. After mounting all of these and taking a look, I see a reasonable bite across the length of the clamp. I have never had the clamp fail to engage. That said, I've always checked the stability before letting go of my camera body. Some plates have a larger surface area interface across the length of the clamp than others. The Arcatech and Hejar are probably the best. I can see a slight additional thickness across some of the others, but not to an excess.
Based on this, I do wish that there would be a better defined Arca-Swiss interface standard. What all of this comes down to is the acceptable engineering tolerances that the various manufacturers use.