Gears (as on some Volvo's, IIRC) or a pushrod engine are inherently more reliable and avoid having to decide between the above flawed alternatives.

I've only rebuilt 3 engines. A BMC B series 1800 (pushrod OHV), my Skywing (sub litre 3 cyl rubber band OHC), and a Honda CB clone (125 cc 1 cyl cam chain OHC) but that was enough to rather put me off OHC engines

Cam chains usually don't drive anything else but the camshafts and tension cogs, but if there is an intermediate shaft (looking at you, Porsche) there can be additional risk of bearing failure. Chains can cause engine failure indirectly because their plastic guides can disintegrate. Cam belts have no guides, but have tension pulleys which press on the back of the belt. The pulley bearing can stiffen, partially seizing and then frying the belt, or causing the crankshaft toothed wheel which drives the belt to wear then strip the belt's teeth before the increasingly stiff bearing actually seizes. Stupidly (or perhaps deliberately for future revenue), some designs make the cam belts also drive the water pump, doubling the risk to belt integrity. But on the whole, good designs with good quality parts hardly ever fail. I have never had an original belt fail in several hundred thousand miles of engine life. (Passats, Focus). Belts can usually be inspected for cracking, crumbling or swelling (Mrs F's Peugeot 2008 has an easily made width tool for this - PSA had a historic problem; apparently supplied with poor quality crumbly belts from DAYCO for its early BIO engines)........

www.honestjohn.co.uk/forum/post/index.htm?t=52783

Maybe we had it right with the push rod operated valve system as in the 105 E Ford Anglia which went on to power several Cosworth race engines.

Or, at the other end of the power spectrum, the cog driven camshafts in the fabulous air cooled boxer 12 of the legendary Porsche 917.....

www.stuttcars.com/porsche-917-technical-details/