Clashfinder is accurate for the schedule you enter: if two sets share a start time on your timetable, Radar flags the clash. It cannot invent unpublished secret sets or magically know a delayed change on the main stage.
Treat published lineups as the planning source of truth, then re-check the official festival app or boards on the day. Times slip; stages swap; weather reshuffles bills.
Radar's advantage is the loop after the weekend: log what you actually saw. Your diary stays accurate even when the printed grid did not.
If you are comparing standalone clashfinder sites to Radar, ask whether the tool still remembers your sets in December. Planning accuracy matters in July; diary accuracy matters forever.