I have had a number of projects that were absolutely successful from a technical standpoint because of FP methods. No one knew except team members that were unfamiliar with the techniques. While they were not full converts, many "saw the light" and reduced mutation and wrote purer functions after that.

The business itself was oblivious to why the project was successful. Or how we were able to extend it so quickly and retain stability.

Even on that project, 90% of the difficulty was social/political. Just as Java allowed average programmers to write above average code, the same thing can be said of FP.