Warning: "Using fall-back BibTeX(8) backend:(biblatex) functionality may be reduced/unavailable."

The warning is issued when using the BibTeX (or BibTeX8) back-ends as these are more restricted in their functionality than Biber is. As such, some biblatex features simply won't work unless Biber is in use, and others will use a 'fall back' approach with reduced functionality.

Depending on your requirements, BibTeX(8) may be fine. For example, in my own subject area, chemistry, the requirements for bibliography formatting are simple and BibTeX is able to produce the same output as Biber. However, this can only be known by the user: biblatex issues a warning as this informs the user they have to check that they get the output they want.