While many companies and organizations, including PMI, find a hybrid approach of waterfall and Agile delivery acceptable, most are unwilling to blend scaled Agile approaches for their transformation journeys.
One thing I have found as true, there is no cookie cutter approach, no silver bullet for every organization or team to follow for guaranteed success. In fact each must be evaluated for what will work best for them, based on the culture, organizational structure, history of the teams, and experience of those developing the products.