Joe, 

I had the 1000 Newton thrust column pairs in use for almost 8 years and they never became unsynchronized.  When I shipped them to their new owner in New Jersey, he tested them like I did with the new ones.  The two columns were sitting on the floor and when he raided them to their maximum extension, one column was about 1mm taller than the other.  After he ran the initialization routine, both columns were the same height.

As far as lifter runaway, the 1000 Newton columns take 25 seconds to full extend the 500mm (19.7").  The 2500 Newton columns take 50 seconds to full extend the same amount.  I doubt of your safety override mechanism would be needed, but if desired, it could be added to the system.

Yours truly,

Jack Huerkamp