Indeed. I've never seen one 'worked loose'. Improperly installed, improperly sized, not installed yes, but never worked loose.
Hmm, interesting. Can't see that happening if the clip is the correct diameter relative to the hole, but ok. Be that as it may, the OP thinks there was no safety at all in his case:
"My guess is that the yard forget to safety the clevis pin after installing the new radar and wind instruments."
Well when we were at the dock trying to get the rigging undone, pliers had to be used to get the other rings off. So yeah, maybe we sailed for a couple of months without it or someone took it out. Guess those are the only other scenarios. The ring was on the deck after the mast came down. None the worse for wear as well.
Yeah, use circle rings if you want to roll the dice. I have had the "work" all the way on on several occasion. I only use them there I can inspect on a regular basis or actually see them every day. Ok for lake boats, no place on ocean going boats.
Circle rings can and will work loose. I think the gremlins find them much easier to undo. Personally I use cotter pins and then wrap rigging tape around them to prevent snags. I also do a periodic check of all the single points like rigging connections even getting out the binoculars to make sure all the safeties are in place. I have seen cotter pins with one tang barely bent over at perhaps a 5 to 10 degree bend and thought to myself, "it won't take much to have that pin work loose" Its all a matter of personal comfort level. I also ran an extra wire of each gauge wire when I built the new engine wire harness back to the engine control panel on my 30 footer. Call it overkill if you want but I always keep in mind... if something is going to fail ..it's going to happen at the worst possible moment. I don't want to find myself on a lee shore against rocks and have a circlip work loose *TINK TINK TINK* and *POP* *BANG**CRASH* down comes the rig and can't start the engine because of lines loose in the water and ...opppss hear comes the surf line and then I'm on the evening news looking forlorn cause I decided that cotter pins were too much trouble.