More weight = bigger springs, more stress on subsequent parts.

More weight + bigger, heavier valves + bigger springs = More + More + more stress on rocker shaft, rocker bolt threads, cam lobes, tappets etc etc.

Having said that I can't say I've ever run these. And I haven't heard of any catastrophic failures as a result of people running them.

If they reduce the rev limit and save the wear on the valve stems and generally promote longevity then I might buy a set for my stock A15 (when I have money )