engineers are more free to design combustion chamber shape.
I'm going to have to disagree with that. When somebody comes out
with a 4-valve pushrod hemi, you might have a point, but it hasn't
happened yet. DOHC allow a lot of chamber design flexibility without
restricting the spark plug to a non-ideal location. As far as
two-valve setups go, there's not much you can't do with DOHC.
Incorporating a pushrod valvetrain into a proper hemi head involves a
lot of complex pushrod and rocker nonsense that makes it pretty
complicated, and asks for trouble at high RPM. (All that
reciprocating mass also eats power, which no one has pointed out
yet.) Off the top of my head, only BMW, Chrysler, Lancia, Renault,
and Toyota have gone to the trouble of putting hemispherical
combustion chambers on a pushrod engine.