Theres all manner of considerations in engine construction... metallurgy is a great part but techniques in "building" our alloys, machining techniques, casting and forging techniques, internal tolerances and oil control/chemistry are a few of the other considerations.
In the case of aluminum heads. Thermal stability is much different than other metals used in the engine. Aluminum heads are a PITA to seal to a cast iron block... enter the torque to yield bolt and multilayer steel shim headgaskets (not to mention some pretty tight tolerances in regards to machining finish and surface flatness.
Aluminum blocks are almost a conumdrum.... currently, technology is still having us use iron sleeves. An aluminum piston just wont live in an aluminum bore. Speaking of pistons... the modular engine pistons are hyper-eutectic but they are still aluminum. Gone is the deeper skirt found on old technology engines (hence the prevalence of piston slap on cold engines) but there is a (I forget the high-tech term) super slippery insert on the major and minor thrust surfaces of the lower portion of the piston.
Crankshafts... cast iron is still a bonafide material.... relatively cheap to cast or forge close to finished dimensions.... machining and balancing required. Forged steel, much stronger, same weight... much more expensive to build.... interesting note.... they can be machined from a great big steel billet to finished shape... or they can be machined from a half sized steel billet (looking like a flattened out crank) and twisted to shape. Dome properly, the latter yields a much stronger crank but they are difficult to produce and spendy, spendy, spendy.
Todays engines are more "purpose built".... Grandads old Buick Roadmaster had this great, huge lump of cast iron in the engine bay.... You could abuse this thing all weekend, take it back and Grandad would never know the difference. Todays motors are very good, but only if we don't exceed design intentions.... overheating will kill a modern motor very quickly..... ditto some of the other abuse we seem to be able to hepa on these machines.
For fuel economy, the gasoline Ottocycle engine has gone about as far as it can.... fuel management is the big concern but emissions reductions sometimes means we can't expect less gasoline to pass through the motor. The Escape hybrid uses an (IIRC) Atcheson or Atkinson cycle engine... same design as the Ottocycle but the valve timing has been drastically altered to reduce fuel consumption.... However, it also reduces the usable power band of the motor. As the CVT transmission gains acceptance, I think we will also see even more changes to the engines powering these vehicles.
My own outlook sees the diesel engine as being the "next big thing"... new management systems that have them running quieter, sulfur content in fuel is being mandated at lower levels, reduce particulate emissions and easier fuel computations being some of the driving factors.
For the most part, it is a complex dance between mileage, power output and tailpipe emissions.
FWIW.... in our harsh northern climate, we see more new technology motors lasting 300,000, 400,000 and even 500,000 kilometers than we ever saw with old technology motors.