The principle is simple and fine, chop up a bunch of mortgages from all over, with the same risk profile and pool them together.
That way local environmental factors don't cause the whole investment to go south. Basically raid for investment
What is a problem, and one that still hasn't fully been tackled is the industrial scale fraud that sold delinquent, or knowingly under par mortgages as high quality instruments.
Through all the use of fancy words, every seems to have got the impression that commoditized mortgages investments are complex, using words like "tranche", "proprietary instruments" and blended risk.
its really not, get a bunch of mortgages, shove them in a blender and make mortgage sausage.
So long as there isn't widespread and systematic fraud, then everything will be fine.
However considering that there haven't been large scale arrests, I don't hold out much hope.