For me it's about the speed of understanding what the code does. Because filter and map are very constrained in what they can do they quickly tell me a lot about the shape of the computation I'm working with. In contrast reduce is much more flexible and so I need to do a much more detailed analysis to just answer a basic question like "is the result a scalar or another collection?".

Exactly. If I need just filter/map/some etc., I use it. But if I need a combination of more than one, that's a job for reduce().

Chained filter and map don't necessarily iterate multiple times. They certainly can but depending on how things are built they very often run as a single loop with the operations chained.