Aluminum has a much larger specific heat capacity than copper, it may compensate some of the disadvantage, that's why mainstream brands rarely make copper heatsinks.
To be honest, I am worried that other cases with similar structure and size (DAN C4, Sidearm T1)may make the competition really tough if the Mjolnir doesn’t come out soon.
Only Asrock announced one.
But it has been approved that some B450 boards can enable PCIEx4 when paired with a 3000 series CPU. The MSI B450 mortar is one of them which is MATX.