> combinations of .map, .filter, .reduce, etc are often less readable than doing the same thing in a nested loop.

I often find map/reduce/filter easier to read when using named functions (or lazy data structures ) for intermediary results - depending on the language/runtime that might imply more allocations - or not.

Eg pseudocode:

Integers.filter(//non-obvious prime sieve).sum()

Vs

Primes = Integer.filter(//non-obvious prime sieve)

Primes.sum()

Or lifting the anonymous filter to primes()-filter:

Integers.primes().sum()