I incubated two different orders this year, one was 100% bunk, not a single egg hatched I believe out of something like 30 eggs, can't recall the exact number off the top of my head... Several had detached air cells but I was still surprised that none made it... Out of that batch only a few even started but blood ringed within a few days time...
On another order I set 8 and 7 hatched, so 88% the one that didn't hatch never started and appeared to be infertile, no signs of life at all... These eggs arrived perfect, no detached air cells or any damage...
I hatched about 95% with my own or locally obtained eggs this year, as a baseline reference...
I firmly believe that it's all a true roll of the dice with shipping eggs, you either get a package that was handled gently with no adversity or you get one that was handled bad or was exposed to adverse conditions... There are just so many variables, from truck freight, to plane freight, to storage at multiple facilities, to handling in general, so many chances for things to go wrong...
You should also consider the persons success rate on local eggs, for example when someone says they got a 50% hatch on shipped eggs, if they only hatch at a rate of 50% with local eggs that is entirely different from the guy that normally hatches 90%... The guy that only hatches 50% with local eggs probably has little chance to do better with shipped ones, so it's sort of misleading as his 50% hatch rate might actually be due to his/her incubation procedures not shipping...