> Thats because if I simply used map, I would get an array of arrays that I would then need to concat into a single array.

No, it should give the same result. One single array with concatenated paths. Besides it also use path.join which is same then simple string concatenation. 

> Can you tell me if you see any issue with using async?

  1. The function is doing synchronous work, there is no need to use async (And it's confusing if you do).
  1. Bring in unnecessary dependency increase module size