Weight, complexity, support & resale value, all these factors are why auto engines haven't flourished at the GA level. Cessna & Mooney I think dabbled in it, soon fell by the wayside. I like my simple air cooled 180HP, Lyc 4 banger, it's easy to maintain, parts are available at any milkbar around the corner & every LAME knows about them far & wide. With Electronic Ignition, a good engine monitor display & a known history I see little reason to make the powerplant complex like that of an Auto engine. A Vans RV8 powered by an Auto donk recently sold in flying condition for around half of what it would have fetched if it where properly set up with a 200HP Lyc, there's another reason!
Each to their own as choice is there but not taken up in large numbers.