The benefits of the hemi are pretty much undisputable. The only drawback is the cost of manufacture. They are considerably more expensive to build than the typical wedge engine, mostly due to the double rocker shafts. Interestingly, Chrysler also built a related but less expensive "Polyshperical" V-8 with a single rocker shaft. The Poly engines had a valve arrangement very similar to the Big Block Chevy. In the early 1970s, Chrysler tried a lower-cost version of the Hemi with ball-stud rockers instead of shafts. It was ultimately rejected, as the cost of manufacture was still too great and the need for such an engine was on the wane.
With a 4-valve per cylinder layout, in order to fit the valves, the combustion chamber is altered somewhat, adopting a pent-roof chamber layout. Hope this helps.
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