They ranged from somewhat complex to highly complex. Ultimately though, a CDO like any bond contract, is just a legal document outlining a waterfall of cash flows based on ratings triggers, collateralization levels, defaults etc. So as long as you read the document carefully it's more legal than financial to run a series of cash flow scenarios (in whatever complex or simple way you choose to generate them) through the structure. But they were all bespoke, yet often similar enough, that i think many people didn't pay enough attention to the minutiae. Also, typically a CDO was initiated by a hedge fund, un or similar who would own the riskier tranches like the equity, with the more secure tranches going to Asset Managers/Pensions and Insurance companies. So you're now dealing with a complex highly nuanced waterfall structured by the owner of the riskiest tranche.