What is the reason for the stability of RNA-RNA hybrids? Why are they resistant to RNases?

Answer 1

Don't you mean RNA-DNA hybrids? RNA-RNA is double-stranded RNA, not a hybrid.

Most RNases break down single stranded RNA. This is what allows RNase protection assay..

RNases, like all enzymes, have a catalytic site that recognizes a particular shape. An RNA-DNA hybrid duplex would not fit into the business end of the enzyme so it cannot be cut. However, there is an enzyme that specifically degrades RNA-DNA hybrids. RNase H is capable of recognizing hybrids and specifically degrades the RNA strand.

Answer 2

If you are referring in vitro, probably because the RNases chop RNA 3'->5' and having double strands (not only intermolecular but also intramolecular secondary structures) can hinder the processivity; single stranded is probably a better substrate from a steric hinderance point of view.

In a cell I think it's the other way round. Double stranded RNA is targeted for degradation but I guess you have more components acting in that pathway. E.g. I think that small regulatory RNAs work not only by inhibiting the ribosomes from binding but also targeting the mRNA for degradation,