The system of tiers and their inner levels, while useful for non-serious debates, is mainly biased and inherently takes away any validity a power scaling may have.
Its weaknesses arise as soon as a small nuance appears, hence why the basic hierarchy of YC1-YC2-YC3 is constantly redefined to include characters who either belong to the categories of "YC" (like Marco, Roo, Snack...) or are narratively put around that spectrum (Doflamingo), and then labels such as YC0, YC4, YC1.5, YC1+... are required to a point that you wonder what is a "YC1" to begin with if there's only one or two characters maximum agreed as such. Either this or a character is forced into one category because of non-fighting related reasons, hence why many users keep grouping the likes of Smoothie, Jozu, Queen... as YC2 because of their rank.
The second problem is that, by calling a character a "high tier", a "top tier" or whatever tier, people are willing to abandon themselves to category bias. Just like we tend to see extincted dinosaurs as temporally closer to each other than to humans even though tyrannosaurus were closer in time to humans than to his fellow dinosaur stegosaurus, when we build our thought on tiers we eventually either overrate or underrate characters who we perceive as inbetweeners, or hard to assign to a static category, and who are round up for good (the higher tier) or bad (the lower tier).
Obviously, the only proper way to attempt a power scale (which remains a bit absurd in this series, but well) is to assign a character to a place throughout a spectrum of strength from lower to higher (e.g.: 100 > 99 > 98 > 97...), but this, albeit way more accurate, is harder to establish and manage; which is why the favorite hobby among the fandom is inherently and for its most part a waste of time.