It also depend on the prior handling of the particular plant. I plant the baby offsets in small gravel, even without RFUG, and they do fine and will eventually form a carpet if undisturbed (big IF). I cannot substrate plant rhizomes which have be grown water-column only, they will rot.
Clarification: Java fern has no roots, they have holdfasts (comparable to macroalge in SW), which have no absorbtive function.