Part of the problem with the mortgage bubble though was that no one (other than a very select few, that were shorting them by buying CDSes) really knew what was in all the mortgage bond derivatives, especially once they got put into CDSes and then re-packaged into CDOs. By the end of it even the people that knew what was going on had to spend tremendous amounts of time figuring out what specific mortgages were in what packages, whereas under the new proposed rules at least all that information will be available up front (in theory).

And as sibling comments say- once you know what's actually in them, you can use your own risk models. If you don't know what's in them, tough to do that.