.....The Spinlock Deckvest is very popular, but not approved. It uses a cow hitch to attach the tether, but the vest has a built-in cutter. It has thigh straps. I have added a crotch strap to my pfd.
I'm a fan of the Spinlock Deckvest and its integrated spray hood, s-cutter, lights, thigh straps (very important) etc. As the OP pointed out, there is a lot of personal preference to this one and I find the Deckvest to be most comfortable rounded neoprene instead of sharp edged nylon. Personally, I see no circumstance where one thinks a harness is necessary, where a pfd wouldn't be. Maybe its just me. I can see one wanting a pfd, but not necessarily need to be tethered in, and there is no downside to the deckvest. Just don't clip in.
Not sure what you mean by needing a cow hitch. You could use any regular tether you like on the deckvest. I prefer Spinlocks, which has an ordinary opposing movement clip at the harness. II'm in the camp of not trusting a quick release and prefer to cut it away, if necessary. I would rather focus on setting up the jackline, so that being dragged in the water was an impossiblity anyway. That's fielders choice as well.
I never did really research why they are not USCG approved, but many European safety items just refuse to go through the US bureaucracy. Some moto helmet companies refuse as well, but doesn't make them poor choices.
As the Spinlock vest are not approved, you do need an approved pfd for yourself squirreled away somewhere to comply.