My understanding from this accident and the Glen canyon dam accident, is that they are not very good at concrete in contact with rushing water.

In this last instance, the rushing water is eroding the dirt under the emergency spillway so it will end up forming a bridge. the emergency spillway is a mini-dam, it will just fall if it's not supported. And if it falls, the rushing water will erode the rest of the dirt, lower and lower. The worst case scenario is a complete dam accident (uncontrolled drainage of the lake) that would not even involve the actual dam.