There might be few reasons your reviewer didn’t like the transformers in map step:
-For consistency sake, i.e. companies have there own best practices which defines the way their developer should code. This includes naming conventions too.
-For readability purpose because it is easy to walk to through the flow service if services are used separately rather in map steps.
-There is no guarantee of the order transformers will execute services in MAP step.
I personally like using transformers in MAP step because it reduces the amount of code and there is no performance penalty that I know of. So I guess it all boils up to your preference and way of coding.