The success of any CMS project depends on the day to day usage of its users and how the CMS actually works, in detail. For example, one of your requirements would be to have flexible workflow capabilities, a very advantageous feature. But how does this flexible workflow feature actually work? In reality, there are many different implementations, starting from a single drop down status changes all the way to developing workflow foundation frameworks. At the end the answers are in the details. Asking "how" questions with the aim of building the CMS strengths and weaknesses will help you make the best choice.
One approach would be to ask about how each CMS can meet your "must-have" requirements or needs. And evaluating your choices based on "how" the CMS meets your need.
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