Magnesium riser are cast out of a mold,or more correctly,a die.AND yes,they are not as strong as billet aluminum risers.I have seen them break in 2 at the grip.Glad I wasn't hanging on t it.
BUT,let me add that the technology today is leaps and bound above what it was just a few years ago.The newer hoyt magnesium riser are practically bullet proof.
Aluminum risers are cut out of extrusions,not forged.I would personally like to see a forged, aluminum riser.That would be the most stable and strongest riser one could get.
I believe forging will be the next technological advancement in riser design.Might even be a dropped forged riser.
Arthur,where you at? I know you have some valuable input on this subject.