I have not been blessed with the failures and have been working a "naked Berea Sierra" parker style for several minutes. This is a chrome version that I have had for about a hear and has yet to be assembled with a finished barrell.
The finial piece fits snuggly over the top of the transmission with a fairly wide range of tube lengths possible -- +/- about an 8th of an inch. If there was a problem with the friction fit at the finial end fitting around the top of the transmission, it would be easily and (for me) pretty noticeable.
The other spot where I think I heard qutestions is the connection between the transmission and the lower section (nib end) - the threads are few (3) and appear to be tapered. Unless the transmission is securly tightened the lower section is "rattling and wobbly" It only took less than 1/8 turn to make it feel insecure.
I suspect those who are having sierra pen come apart are having the threads loose and the pen becomes top, bottom, refill, spring.
I went back and looked in stock and have about a dozen finished ones in stock with the double twist action. When I assures the threads were firmly set with the finial turned clockwise to a firm set, they all worked the tranny with a counterclockwise turn but with a noticable extra twist, the transmission would unthread.
I do not doubt that those who are having problems, are having problems, but it does look more torwards tolerances than a general design flaw.