At what point would you step up to ferrea hollow stem valves

I don't see the downside of a well made hollow valvestem.
3/8 hollow pushrods are the norm and they're longer/more slender and no one thinks twice about them. They also see the same forces as the valve does.
It's possible to drill a valve too deep and make the head weak, which is why I say "well made".
Less stress and less chance of spring resonance by lightening the valve seems like a win in all cases to me.