The Problem with Fear Scaling
A large portion of Devil scaling in Chainsaw Man comes from the fact that Devils get stronger the more feared they are. This is supported by the vast discrepancy between Devils such as the Gun Devil and the Tomato Devil; the first has impressive speed and destruction feats, whilst the second gets killed by human Denji using what is effectively a normal chainsaw.
What bolsters fear scaling greatly is Chapter 148, where Yoshida is unable to dismember Asa despite having cleanly severed off one of her arms seconds prior in Chapter 145. Yoru also finds herself surprised by both her new speed and the fact that activating her weaponization became easier. All because of a pretty minor boost in the fear of war between those 2 chapters. This establishes that a Devil's strength, stats and hax, are all boosted by fear.
This boost is extended to other Devils such as Pochita and the Primal Fears, the latter of which have fewer feats of their own to scale them, so this grants us something to work with. However, I have to question it since we have a glaring issue: the strongest Devil, Death, does not abide to this rule. Consecutively, across several chapters, Lil' D is hurt, made to bleed and speedblitzed. It makes little sense for characters like Yoru or Pochita, whom in both Chapter 159 (where Lil' D gets dropkicked and bleeds) and even Chapter 218 (where Lil' D gets decapitated) after their empowerment, to have the AP to hurt her and the speed to blitz her given the rule. Death not reacting to Pochita and getting beheaded is embarrassing, but her having to be pushed aside by Yoru to avoid weaklings like the Nail Fiend and Katana Man is even worse. In both cases she has all the reason to dodge on her own since any deaths she suffers brings the activation of her powers closer, yet she does not, meaning she probably cannot move that fast. And that's assuming those weak characters COULD hurt her when, according to the rule, they should be breaking their arms trying to damage her.
Finally, Death Devil's power weakens her own pawns, something unique to her and not demonstrated by the other 2 Horsemen with similar abilities, Makima and Yoru AND contradicting the fact that fear scaling affects a Devil's abilities too, as seen with Yoru's touchless weaponization post-fear boost. Even early Part 2 Yoru could create powerful weapons such as the Uniform Sword and neither Makima nor Nayuta weakened the people and Devils they controlled. On top of that, her necromancy ability has her pawns sometimes disobeying her along with other unnamed limitations. By far, Lil' D has the weakest showings of any of the Horsemen stat-wise, even Fami was at least somewhat trying to keep up with Fakesaw.
Although Lil' D is the worst offender, there is also Darkness Devil who was damaged and staggered by Makima despite the immense stats it should have. Falling Devil gets away with it since, as stated, she is weakened by Lil' D's ability, though one has to wonder just how much of a nerf that ability gives in order to make a Primal Fear a bullet victim.
The meta explanation is Fujimoto just butchering Lil' D by thinking of the Death Devil twist only after 'Fami' had been introduced as a character long ago. Basically, he had intended for her to be Famine but switched to her being Death too late in the story. The messy and non-sensical nature of her plans supports this, so the narrative is on the side of this explanation too. She could also just be an outlier and one exception (2 with Darkness) does not disprove the rule, but I would like to hear your explanations too, especially if I am missing something.