Depending on the business-application, fixed-width formats are fantastic because they allow for genuine O(1) random-access to records without needing to precompute an index first. Writing parsers for fixed-width formats is also much simpler, for example, because it eliminates ambiguity around null-terminators vs length-prefix in text/string/array data - for resource-constrained environments it's great because you can guarantee you won't need to dynamically-allocate buffers. I feel the only real arguments against fixed-width records are concerned with wasted space - which are mitigated if your system lets you compress them somehow - because they'll typically compress very, very well.

Sometimes, fields can turn out to be too short. I have received a few letters from business with my last name truncated, because somebody way back when decided eight or nine characters are definitely enough for a surname.