Worked for two airlines that had prior and present (seconded) BA co-pilots, and they all seemed superbly trained...if just slightly full of themselves, but hey, why not, BA has a pretty good record, overall.
They suggested in their 'new' company that the First Officer had the duty...actually the right, to call STOP at anytime, and the Captain should obey, period.
Oddly, the 'new' management didn't agree, and these guys were
told to keep their opinions to themselves, as in...our company way, or the highway.
So, they clammed up and did as they were told.
Having said this, they were pretty good stick and rudder guys/gals.
Also, very good technical knowledge about the equipment.
OTOH, the very best seconded First Officers that I have ever flown with came from...oddly enough QF, many years ago.