A character that has more skill can turn around an advantage better than someone with less skill than makes perfect sense to me. I disagree with Skill 5 getting less penalized from a bane, though, as the bane could cancel out the skill while a full bane would only make the 10 half as effective.. A character that has a skill level 10 will benefit more of a boon than a character that has skill level 5 and a character that has skill level of 10 will suffer more of a bane than at character that has skill level 5.
I see the phrase, "difficulty level," and this might be part of some miscommunication.
In D&D where you:
- Set a DC
- Add or minus a skill bonus
- AND THEN use a boon or a bane
- Roll a match or higher number
- Use the PCs skill levell
- Use the bane/boon only when something is very hard or really easy
- Roll a match or lower number
Advantage/Disadvantage is not a Difficulty Level on its own. It's for outside factors like weather, extraordinary power, or conditions. And thus a different philosophy compared to setting up a difficulty level.
Not saying more granular penalties/bonuses to set up a difficulty range is badwrongfun, just different than what we already have.