your info is wrong. Jedi has had G70 hot dip galvanized since her launch in 1994 and Dashew's original Sundeer before that. It used to be special order from ACCO but it's now even listed in the West Marine
My 3/8" G70 held Jedi (25 tons metric, 27 US) during hurricane
Ivan with 120 knots sustained wind
and without any visible damage. At the same time, the 5/8" shackle got elongated but didn't break either. We replaced the chain for a new one and upgraded to 3/4" shackle.
All the Sundeers have G70 chain. The 56' and 60' versions have 5/16" and I never heard about any trouble with the chain.
You are right that it is much harder (which is another way of saying more brittle) than G40 but it is also much stronger, taking much more tension than grade40 before breaking.
The only negatives are higher price
and re-galvanizing isn't advised.