I think I posted somewhere in all of this that I'd done a quick experiment with the UCR, leaving only about two or three inches of the constrictor section tightened with the remainder left loose. I then sat in the hammock. There was an initial slip, but then the remainder of the constriction section gripped -- not all the way to the end, but I think I now know why* -- and held firm. I've never had my UCRs land me on my bum, and never had one slip after being set. I think both slings and UCRs are both great, but I'm not particularly concerned about slippage with either of them. I now believe that the securing hitch I used on the loose end (I now use a rolling hitch instead of the slippery half hitch) is the UCR analog of the lock in a locked brummel -- it keeps things from working loose under light loading. (IMHO... I'm no expert on rigging)... and I had one episode of slippage with the URC when I forgot to tighten it up properly.
* Re the loose end of the UCR's bury -- I need to think about the physics of this some more but, after a little head scratching, I convinced myself that the static friction is distributed as a decreasing exponential along the lenth of the constrictor section. Given a long enough section, for a given line load, the 'loose' end of the constrictor feels hardly any force; it's not gripping, because it's not induced to. Add enough load, and more of the constrictor comes into play. Add too much load, and the entire constrictor will be insufficient to hold.