Learn Finnish: Hyvä kasvatus on vaikeaa, jos lapsen ruutuaika on liian pitkä. - Good parenting is difficult if the child's screen time is too long.

You could, but it changes the tone and meaning slightly. jos is a simple, direct 'if'. sikäli kuin means 'insofar as' or 'to the extent that'. Saying sikäli kuin lapsen ruutuaika on liian pitkä sounds much more formal, academic, or restrictive than the everyday observation presented here.

Why is the adjective vaikea in the partitive (vaikeaa), but pitkä is in the nominative?

This is because of how their respective subjects are viewed. kasvatus (parenting) is treated as an abstract, uncountable concept here, which requires the partitive complement vaikeaa. However, ruutuaika (screen time) is treated as a specific, measurable whole, so its complement adjective pitkä stays in the basic nominative case.

Since this is a conditional sentence with an 'if', why don't we use the conditional mood (olisi) instead of on?

The conditional mood (using the -isi- marker) is reserved for hypothetical, imaginary, or unlikely situations, which would translate to 'it would be difficult if...'. This sentence is stated as a general, real-world fact rather than a hypothetical scenario, so it uses the standard indicative mood (on).

Does kasvatus always mean 'parenting'?