Mopar Action SB head article

Aluminum heads are much more suceptable to warping if they are over heated. Also because of the difference in expansion rates between an aluminum head and an iron block its harder to seal. Most all aluminum head iron block production motors have gone to torque to yeild head bolts to improve the head gasket seal by having a much more consistent clamping force. If you do blow a head gasket on an aluminum head the softer metal is much more likely to be erroded away requiring the head to be surfaced.

Also, all other things being equal an aluminum head engine won't make as much power as an iron headed engine because it transmits heat away from the combustion chamber faster. That is why you need to go up approximately 1 point on compression when swapping to aluminium heads.