This is what puzzles me about GM…they have excellent V-8 engines, but many of the V-6 engines have all of these problems. As for mating an aluminum head with a cast-iron block: this has been done for years (M-B has done it quite sucessfully). It is true that there will be some extra flexing of the head gasket, because aluminum and iron expand/contract at different rates. If you begin to get VISIBLE leakage, then already your engine has been damaged (the highly corrosive coolant will pit the cylinder walls).
FORD had a similar problem with their 3.8liter V-6. The fix is: high-quality asbestos/steel head gaskets, and correct torquing of he head bolts.
Why GM can’t do this right is a mystery to me!