'hard' should also be an accepted translation for 'vaikea'
Hard should be accepted. Probably more commonly used in ordinary conversation.
Can someone please explain why the expression "Tämä on liian vaikeaa" uses the Partitive and the expression "Tämä on liian painava" uses the Nominative?
I'd say it's because 'vaikeaa' reders to something uncountable and 'painava' refers to a countable thing.
It might be hard to distinguish between hard (= not easy) and hard (= not soft).
Hard material: kova
Hard/difficult (ie. subject) = vaikea