there have been a lot of 747F crashes compared to the vastly more numerous pax versions
I think this is indeed a very interesting point. There are a lot of aircraft types which are operate in large numbers as freighters and pax aircraft (MD-11, A300-600, 767, 757, 727...), but for none of them the difference in accident statistics is as striking as for the 747-400, which has an outstanding statistics as pax aircraft (especially compared to the -100 to -300 model) but a lower than average one as a cargo aircraft (basically no difference to the -100 to -300 models). This should tell us something. Are the typical freighter issues (fire, load shift) increasing with size? Are the 747-400 freighters that crashed especially old / high cycle machines? Are these aircraft especially cheap on the market, so that a lot of them are operated by "less quality" operators? Are these aircraft maintained using the huge amount of used parts from the high number of retired pax aircraft? Is it a special type of pilot flying these planes? (on the other hand, many crashes are not pilot error...) Are they operated closer to the limits (e.g. landing weight)? For sure there is something to learn from the numbers.