The lag is the problem. A 13F published in mid-February describes 31 December, by which point the manager may have exited entirely. It also shows only long positions in listed equities and options: no shorts, no bonds, no cash, no foreign listings, so the picture of a hedged book can be badly misleading.
Used carefully it is still useful for spotting new positions, concentration, and which managers cluster in the same names. Used as a copy list it is a lagging indicator with survivorship problems.
Example: a fund's 13F shows a new 4M share stake at an average December price near $27. By the time it is public in mid-February the stock is $41, and the fund's current position is unknown.
Related: schedule-13g, schedule-13d, activist-investor, form-4, survivorship-bias