Collars and ferrules are threaded. One reason is the threads are the clamp that holds everything in place till the glue dries......forever.
In almost 30 yrs of doing it this way not one collar or ferrule has cracked.
As far as evidence...I've seen collars that have not been threaded separate from inside against the wood.
Why? Who knows....bad glue, wrong glue, poor construction technique, etc etc
I have never seen one that was threaded separated on the inside.
Does this mean all slip fit collars are going to fail? No, but why give it the chance?
It certainly is a personal choice as this debate has happened several times since my time here at azb.
It all boils down to Peace of mind with what you are doing.
As far as "hit or feel".............you'll never be able to tell the difference, imo.