I think you're overstating the difficulty of these characters and are sort of conflating the difference between having tools on a character that are difficult to utilize in neutral vs. making use of the tools that are actually required to have some sort of baseline of success. You don't
need to make use of the tools you listed in order to be successful with them. Filia has airdash cancels and those airdash cancels are pretty integral to her basic combo routes. Yes. But in a game with a macro-able 2 button airdash this effectively just becomes another set input in a chain of similar inputs. It's only difficult if you're trying to manually airdash cancel with something like forward forward. You mention the little drill spikes and how they erupt in set places on the map ... which, combined with what little gain you get for actually hitting them with it, makes it so that most people who use Filia completely ignore them and just run the rushdown game instead. Is she as simple as Fukua? Probably not, but she's still by and large one of the most straightforward characters in the game. Jump, airdash, j.HP -> Repeat till hit.
The Parasoul example is probably the epitome of what I'm talking about though. The trend in players to actually make use of tear setting, manual detonation, and the additional combo routes that come with it is a fairly recent development for her character. And was only made possible with tweaks to her overall gameplay that came with the release of the PC version of the game and Squigly edition. You can play her (and I personally play her) in a far simpler manner. The core of her game is still Napalm Shot zoning (which is literally guile) at fullscreen and j.LP -> j.HP as a form of aerial dominance to start combos. Can you do new and obscure mixups while making use of the additional tools? Yes. But at that point we're talking about the difference of fully utilizing a character to their full potential and finding what tools one actually needs to be comfortable using during the course of the match. All the characters in Skullgirls can be played in a simple or straightforward manner once you've internalized that the name of the game is finding ways to reset your opponent for additional damage. After that it's just magic series, launcher string, into shenanigans.
You don't think Big Band is straightforward? If you're talking about parry then yeah he does have a certain level of complexity but when you realize that parrying in a game as fast paced as Skullgirls is sort of a lost cause to begin with then you realize that DP is a far more useful defensive mechanic. I typically only use parry to jump through zoning because it retains your horizontal momentum while in the air. After that the character becomes jump tambourine special into random armor punches hit confirmed into super. The biggest hurdle with learning the character is that he just sucks and everything instant overheads him.
Do you play on PS3?