vey secure.
I think that most "mysterious" letting go of screws and bolts are in general due to poor workmanship.
We make our set screws as tight as makes sense for a 1/4" screw, and use Loctite on the top of the two screws.
Never had an issue

Even is the collar slides a bit, you will get only a drip

When we crossed the Atlantic in the old boat we had 100 strokes on the bilge pump every 4 hour watch for shaft log drips. (oour shaft was in good shape) When we crossed in in 2008 Milvina (with a PSS seal) we did not pump. We have a small plastic box to catch drips, but there are none. Sometimes we can see some salt around the PSS seal showing some slight leak that evaporates.