I grew up shooting when GN arithmetic was the only thing available. Having flashmetered electronic flash, generally speaking GN are virtually always optimistic, and along with a number of others who did the research and published our results over a decade and half ago, electronic flash GN tends to be 1EV optimistic compared to flash meter readings.
Having grown thru photosensor flash and TTL film flash and now nTTL flash systems, I find the least reliable of all of them is nTTL...sometimes it flashes on full power spontaneously, even with a tripod-mounted camera with flash in hotshoe! Today I often put the flash unit on photosensor automation rather than the unpredicatability of nTTL. When nTTL works, it works well for exposure; it simply does not always work! In comparison, photosensor auto and film TTL are quite reliable. I have gotten shots with TTL that would have fooled photosensor flash.