Okay, let's do the math.

Push rod engines= 1 lifter, 1 push rod, 1 rocker per valve times cylinder.

OHC engines= 1 rocker arm per valve times cylinders and, uhh, hmm that's it!

DOHC engines add one more piece, another cam to the TOTAL.

Pushrod V-8 = 49 parts working to make the valves go ooompity-boompity.

OHC V-8 = 17 parts.

OHC 4 valve per cylinder = 33 parts

DOHC V-8 = 18 parts.

DOHC 4 valve per cylinder = 34 parts

Now which system do you figure is the most reliable?

Granted the length of the timing chain requires usually 2 or 3 guides and a tensioner so add 4 more parts to the totals if you really want to get picky..

I don't want to hear the talk about timing chains. You guys must have forgotten the fiber gear fiasco's of the 60's and 70's American iron. 60-70K was about norm for a timing chain replacement.