The shift linkage controls an internal spool valve that directs hydraulic pressure to the clutch
packs. the valve is held in place by an external detent. The transmission
linkage may be flexing in a seaway causing the transmission to shift into neutral thus the reason for observing while running under the conditions you describe. It could be more likely that the clutch is slipping due to lack of oil or the wrong oil. Changing the oil may help if the original oil is of the wrong type.
Worst case scenario is the clutch pack is worn out and an indication of this would be a burnt smell or discoloration of the oil as well as an increase in engine rpm
with no increase of boat speed,
To answer your question about the reliability
of the Velvet Drive transmission they are pretty much bullet proof but require proper care and feeding.
I hope this helps.