I'm sure that Roman concrete greatly varied in quality. Every batch was an experiment using local materials.The crap that didln't last for 25 year is long gone. All we have left to look at today are the results of successful experiments. And it is a wise thing to learn from it. But to consider everything the ancients built as evidence of their genius disregards the winnowing of time. Good stuff lasts, bad stuff falls apart and is discarded.