From my experience, real engineering is done in a multi-paradigm style. Some functional, some oo, some imperative, etc. As a general rule, functional style is easier to reason about (and test) for smaller pieces of code (ie, pure functions, function composition, etc) but can be harder to reason about for the larger code base. As such, it's very common to see as many functions as possible be done in a functional style.. but the overall system include lots of non-fuctional things.
From my experience, real engineering is done in a multi-paradigm style. Some functional, some oo, some imperative, etc. As a general rule, functional style is easier to reason about (and test) for smaller pieces of code (ie, pure functions, function composition, etc) but can be harder to reason about for the larger code base. As such, it's very common to see as many functions as possible be done in a functional style.. but the overall system include lots of non-fuctional things.