At least two problems associated with the extra hydroxyl group in ribose may be noted. First, the additional bulk and hydrogen bonding character of the 2'-OH interfere with a uniform double helix structure, preventing the efficient packing of such a molecule in the chromosome. Second, RNA undergoes spontaneous hydrolytic cleavage about one hundred times faster than DNA.
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Structural stability is not a serious challenge for RNA. The transcripted information carried by mRNA must be secure for only a few hours, as it is transported to a ribosome. Once in the ribosome it is surrounded by structural and enzymatic segments that immediately incorporate its codons for protein synthesis. The tRNA molecules that carry amino acids to the ribosome are similarly short lived, and are in fact continuously recycled by the cellular chemistry.
Your link is off on a few points. First, RNA can form a uniform double helix; it just needs to be in the A-form, not the B form. Second, RNA can from stable long-lived structures; the ribosome is one. It is true that RNA is hydrolytically labile; but remeber our DNA lasts 70 - 100 years. Shorten the lifetime by an factor of 10,000, and it's still plenty long for a primitive unicellular organism