Agile Vs Waterfall

Whilst there is a huge drive to migrate from Agile to Waterfall, far too often we are seeing what is essentially Water-Scrum-Fall.  As well as proving UAT challenges, we are increasingly noticing resource deficiencies, knowledge gaps with little or no support.  It’s no surprise that projects quickly spiral out of control and external expertise is sought.  What observations and challenges have you associated with this hybrid methodology?