Iron and aluminum expand at different rates when heated… in early cars this caused a tendency for head gaskets to fail prematurely. Modern advances in gasket technology among other things are why engineers have been able to make it work more reliably so the MTBF penalty is not as severe in the later years.
On a slightly different note: the most reliable engines I’ve owned had bi-metal construction, and the least reliable ones had the same block/head construction. 
My Ford Thunderbird Turbo Coupe blew a head gasket: all iron
My Volvo 740 Turbo ran well right up until I sold it: iron block/aluminum head
My Ford Taurus SHO ran well right up until I sold it: iron block/aluminum head
My green Lincoln Mark VIII blew a head gasket on the previous owner (which gave him an excuse to buy a set of ported heads 
): all aluminum
So… ummm… my experiences with bi-metal engines seem to suggest the opposite, but the physics are what they are.