wilbur
Jameel Abraham, who's one of the principals of Benchcrafted, is both an excellent woodworker and machinist, and I'm sure he has thought through the issues you bring up. I've also seen the Benchcrafted wagon/tail vise in action, and given the forces needed for this vise to hold boards, I hardly see how failure of the end cap would be an issue.

Here's a link to the installation instructions. The suggested methods to hold the end cap on include (1) a large dovetail, which is the first choice and I have a hard time seeing how this would fail, and (2) beefy lag screws. Also, the rails of the vise screw into both the main part of the workbench and the end cap from the underside, which provides another mechanical method of preventing the end cap from coming off. I think that if you actually look at the method of installation, any concerns would be laid to rest.