Mechanically the push-rod style motor is less simple just because there are extra parts & geometry involved vs. over-head cam designs. P/R valvetrains have more parts/sub-parts (typically lifter, pushrod, rocker-arm & all the associated bits), and more parts = more mass & friction/deflection/hot-spots/wear-points = generally less rpm/surface-speed attainable than more efficient OHC designs. But the geometry of the blocks & heads (both casting & required machining - really factors-in when considering oil-passages, I think) & cam-drive mechanism, and adjustment of valve clearance are generally simpler & more easily accomplished (or automatic, with hydraulic lifters/lash-adjusters). As for making torque - p/r don't increase the torque a motor produces, they're just generally used in large-displacement relatively low-rpm motors which tend to have more & are tuned for bottom-end torque. All that said, p/r technology still does a hell of a job in many applications, has for decades and will for many more - it's just not as well-suited to very high-rpm operation & is *relatively* 'old-fashioned' technology. Some of the best things are though!
O/H cam & shim-under-bucket is about as mechanically simple & efficient (fewest parts, least reciprocating mass/energy wasted) as you can get, but the manufacturing tech & precision required is a LOT higher to produce the parts (block & heads, mostly). Can be a real PITA to deal with clearance adjustment also, of course - what with having to pull elaborate air-boxes/hosing & throttle-bodies & shit just to *start* removing the camshafts (and usually cam chain/tensioner) + having to measure/calculate/replace shims which MUST be meticulously & correctly done. Believe me, sometimes I've *wished* I could just slide a couple tubes up & be done with a couple wrench-twists, without having to take much if anything off to get to them!
