The un-shrouded (beveled) bolt carrier has an interesting history. Around 1970, Colt identified a problem with uncontrolled automatic fire (hammer follow-down) if the disconnector was removed in its then-current AR-15's. (It's unclear whether the ATF complained to them or not, but I'll bet they did.) As a result, Colt took the AR-15 entirely off the market until it could do a redesign of the offending parts. (Incidentally, that hiatus in the availability of the AR-15 gave a market opening to the Armalite AR-180, which filled the void.)
Colt's solution was the beveled ("un-shrouded") bolt carrier, combined with a notched hammer. In this design, if the disconnector was removed, the hammer would snag the firing pin and hopelessly jam the gun. With these changes, the gun was reintroduced to the market.
Some time later, this whole problem was found to not be as significant as first thought, and Colt (and other manufacturers) went back to the original "shrouded" bolt carrier, apparently with ATF's blessing.
It's pretty clear that the shrouded bolt carrier is more desirable, in terms of reliability. Just be sure to have a functional disconnector.
The key thing with an unshrouded (beveled) bolt carrier is to use a firing pin with a small-diameter collar. If you use the standard large-diameter firing pin, the hammer may still snag on it even if everything else is functioning normally. I suspect that that might be the problem with these Ruger guns -- someone is substituting a standard firing pin for the small-diameter one. Or maybe the mistake was made at the factory -- who knows?