I can only partly see which parts are subject to bias.
Stricter timing windows are more difficult. As skill and practice increase, this becomes less of a factor.
Any kind of reaction test will show you that the more options you must choose from to match a stimulus, the slower the reaction time becomes.
These are the metrics I want to use. Still not subjective, really.
Tournament results don't tell us who is more difficult to play, necessarily, because there are too many variables: people. All the people playing Pikachu before Axe (and I guess Azen and Chu when they messed around with him) were not skilled enough players to take him to the level he can currently be played at.
There are a lot of ****ty space animal players. There are more ****ty space animal players than there are ****ty Peaches.... but that has a lot to do with spacey saturation, and not necessarily how hard Fox and Falco are to play. More people want to play Fox and Falco so you will see more good AND bad players of those characters. If you only have a 1% chance of seeing a top-tier player devoting themselves to a given character, and there are like 5 Roy mains in the country, the odds of them shaking tournament results and bringing that character's potential to light are kind of low.
And yes, our top players that actually advance character metagames are that rare. Probably MORE rare than 1%.
You are right, there's a small amount of subjectivity involved. By the standards I've established, ICs are harder to play well than Fox or Falco. The weird thing is though, because of my skillsets as a human being, they are much easier for me to play than Fox or Falco.
That, however, is why my analysis tries to list things that can be objectively measured. Timing windows, number of options, range and duration of moves, risk factors given what we currently know about punishment options, punishment power of a given character (and then the windows, forgiveness, and risk factors of those punishment options).
Some people think it's really easy to hit buttons fast but they have ****ty spatial judgment, so instead of pressuring with tipped b-air or d-air they opt for frame perfect shield pressure. But that doesn't reduce the difficulty and the window of doing shield pressure, it just says "they are really good at this particular skill, so they excel when playing in situations that emphasize it." It doesn't say anything about the character at all, and subjectiveness can be ignored--especially if you go out of your way to ignore statements like "I think this is really easy lolololol" as anecdotal evidence.
If you want to take stuff like that into account, however, so you can make the subjectivity part of the analysis in a more useful way, you can categorize those elements of difficulty by the type of skill it takes to do them. Motor skills, spatial perception, reflexes, response inhibition, knowledge base... things like these can tell a player which character he/she is more likely to find difficult/easy to play, based on things they tend to be more skilled at.
Why do I play ICs over other characters? Because my greatest skill is maintaining a wide knowledge base in my head, and their massive variety of tricks and gimmicks capitalizes off that. I still surprise even the people I train with most frequently with some of the nonsense I know how to do. ICs don't have many technically demanding elements, and the things that ARE hard tend to be timing-based rather than speed-based (and I'm much better and finding and maintaining timing patterns than executing rapid-fire motor commands). But they are very difficult in terms of risk factor, because losing Nana drastically decreases your power and you can lose her off a single mistake, and then the matchup becomes drastically skewed against you. They are also tough because their hitboxes, despite having decent range and disjointedness, usually have small windows of activity, and I'm not the best at spacng those in the heat of a match. Which moves do I rely on most then? Blizzard and u-air are my staples in most matchups because of how long they last and how much space they control, and how they cover for my lack of skills in other areas. You could do this for other characters and players.
If you wanted, I guess.