CMO and CDO differences?

I understand CMOs to be securities issued/secured by a pool of securities that are collaterilized by a pool of underlying mortgages/loans. So there is basically two levels securitization.

CDOs, however, seem to be more direct. That is, securities issued directly from the underlying pool of loans (be it credit card receivabls, HELs, auto loans, etc). So there is really just one level of securitization.

Is this understanding generally correct? If so, why can’t CMO’s take a more direct approach if CDOs are able to both distribute prepayment and credit risk with only one step ??

And, just to make sure. The reason why credit risk is more of a concern for ABS/CDOs is because they are usually based on non-comforming loans/non-agency loans which are not back by the gov’t like residential mortgages are ?? An exception being, say, student loan based ABS, which are guaranteed for the most part.

Thanks in adance!