A quality flintlock can be every bit as reliable as a caplock when the operator knows what they are doing.
In fact a properly made flintlock rifle will have a faster lock time than the average percussion rifle, though we are talking custom flintlock rifles here as the manufactured flintlock rifles are just not up to par. The vent holes on many manufactured flintlock rifles are just too low to the flash pan. There would be a faster ignition if the vent holes were moved up just a bit (Fire arcs upward).
http://www.nmlra.org
National Muzzle Loading Rifle Association