Nylium has larger hardness than netherrack does, despite being the same thing/material.

That's because the nylium growing through the netherrack helps to hold the structure together thereby making it harder.

Extremely likely intended - it's the same for grass and dirt.

@Galaxy_2Alex Grass block only has 0.1 hardness larger, while nylium has 0.6 larger, causing too much difference.