We couldn't get the basic facts straight. One thought what happened was a risk mentioned in legal DD, only we weighed it wrong. Another saw it as being duped by an active competitor. Another was on the told-you-so-all-along train. This is like hours of meetings with people getting their managers on board to prevent things from being written. General large corp hostilities.
The fun part is, these are all people from different teams, with different backgrounds. We were and still are working together on multiple transactions, with fun and camaraderie. We just couldn't get the blindfolds off.
And that's what leads me to concluding how hard DD is. We've got processess. External advisors. Teams with experience with the work and each other. Then something goes wrong, you try the Challenger-approach and get nowhere.
I think you are dramatically overestimating the quality, depth and especially the diligence of that process.