I think the question of whether the fingernail test is good enough depends on the skater. Some world class figure skaters, skating many hours/day, are happy going many months between sharpenings, according to Mike C. I've known other skaters who do a touch up on close to a daily basis. (And some very picky hockey and speed skaters who do it MORE than once/day.) So different skaters get used to skating on blades of very different sharpness levels. A few of the school figures skaters actually skate on flat (infinite hollow) blades. I think it depends a lot on how heavy you are, and how strongly you "push" into the ice.
As far as factory sharpenings being dull - it is possible that some brands use a very coarse grit wheel, to do it more quickly, and that would indeed be less sharp than many skaters want. But my Jackson Ultima Matrix I (an old model) runners came a lot sharper from the factory than most skate techs routinely produce - you'd need to use a very fine grit wheel to make it that sharp, and relatively few skate techs do that, because it takes more time. They were also extremely consistent in shape. I assume Ultima still does that - at least on their high end blades. Paramount also makes a big deal out of advertising that they do a very careful sharpening job, though I have no experience with their blades. I seem to remember that Skate Science advertised that way too.
I got used to very sharp blades early, because my first good sharpening tech did that, so the fingernail test was indeed not good enough. But I've tried to get used to less frequent sharpenings, to extend blade lifetime, and it is close to what I need. When I skate on an outdoor rink, where ice conditions are erratic, I often need very sharp blades again, or I skid all over the place. (On very soft or very hard ice, I need sharp blades and to push hard.)
I think the fingernail test is a good starting point for someone who has no idea how sharp they want the blade.
I generally use the finger touch test too - but I hesitate to tell other people to do it, for the same reason I'd hesitate to tell other people to pick up broken glass with their bare hands. On a very sharp or rough blade, if you run your finger ALONG the edge, or you use too much pressure, you can cut yourself. What I'm looking for is a certain amount of drag, when I run my finger, very lightly, ACROSS the edge. But I'd hate to tell someone to do that, and have them get infected because they didn't wash the cut.
Most of the time, I just wait until my edges start to skid, when I don't push very hard.
Many (most??) pro shops re-sharpen new blades WITHOUT being asked. E.g., as of a few years ago, Rainbo sports did, even on mail orders. Which means that same-model blades can have very different profiles and sharpness levels, depending on who you buy them from, if you don't ask them NOT to sharpen them.