Reflecting on #CandelaObscura, I think one of the challenges I have with it is that its major "constraint" is your health: the number of marks and scars you have.
But.
As a general rule, as the characters advance, they gain too many ways to obviate, move around, or step in the way of marks. They also have a variety of ways to get rid of marks either over time or through abilities.
Combined with that you need four marks of the same type to get a scar and the system just feels less lethal than I think was intended. The sense of pressure lightens right when it should be increasing.
The game also doesn't give GMs a lot of ways to make the game more lethal.
I think in general:
1. It works best when doing one shots (or, like, three shots), not prolonged advancement. Because the system starts to struggle with advancement _early_.
2. It works best with a group of PCs who are willing to lean in to narrative consequences.
3. It works best with a group of PCs who don't play conservatively.