There's an abstraction level above which waterfall makes more sense, and below which [some replacement for agile but without the rituals] makes more sense.
I think Qs to ask are.. if the nature of user facing deliverable tasks are longer than a sprint, the tasks have linear dependencies, there are coordination concerns, etc
I think Qs to ask are.. if the nature of user facing deliverable tasks are longer than a sprint, the tasks have linear dependencies, there are coordination concerns, etc