I'm very used to horizontal opposed engines, having cut my teeth on aircooled Boltswagens at the same time as British inline four-pots.

One thing about the former is the requirement for multiple heads, a rather precisely machined bit. I would guess an opposed is probably a more expensive engine to produce than an in-line, though the argument falls away comparing them with vee's.

Accessibility is a little more painful in a car, as the heads are up against the wheel-wells. Opposed's are inherently wide engines so the width of the bay needs more thought, especially up front in a Sub'y. In a VW or a Porsche you can just hang it out behind the rears though at the cost of weight distribution.

Still, the lowered centre-of-gravity in an opposed engine is a serious gain, whether for ground clearance without a propensity to be top-heavy in the Subaru or for cornering performance in a Porsche.

Also needing more than just casual mention is the inherent lightness of the horizontal-opposed, especially in its air-cooled form. That's probably the biggest reason the form is so commonly employed in light general-aviation applications.

Just some mental musings, of course...