Many text files use field separators that are not currently supported in Tableau Prep. While the "Other" option is nice, it only supports entering a single character. What if, for example, my input file uses double-pipe (||) to separate fields? What if it does this because a single pipe could appear within the contents of a single field? Also not supported are control characters (non-printable ASCII characters) which do come up from time-to-time.
I'd love to see the text file input options become more robust in future versions.
See this thread for a real-world use case: Ctrl-A delimiter
P.S. the REPLACE() function, which might have been a viable workaround, seemed to choke on control characters when I tried something like REPLACE([field], CHAR(1), "|")