I know a little about this stuff.
Some basic facts:
-Read rate varies from system to system.
-for xc, the timer should always use a transponder on each shoe (or bib tag)
-The MOST accurate systems available are the active transponders (example: JChip or AMB)
-The most accurate transponder on the Planet is the active transponder worn on the bib. The bib tag used by Omega (AMB) has an accuracy rate within .02 seconds of photo finish
-EVERY transponder system can fail. I have been at the World Championships (active transponder) when several active transponders did not read due to interference from nearby video boards.
-For high profile races and ALL XC events, timers must verify the finishers with photo finish prior to releasing results - PERIOD.
My father once told me that the most dangerous part of a car is the nut behind the wheel!