The limit of Essential Nature is a very interesting case, precisely because it is at once both so hard and so easy to violate.
consider this: A magus born in the year 1195 and now it is 1220, thus they are essentially 25 years old. Next year, in 1221, they will be essentially 26. Without doing anything their essential nature has changed!
A PeCo (Or CrCo) spell can change the age of a person in all ways but the essential one. This change is unnatural and permanent it only differs from the essential age in that changes wrought by magic can be undone by magic.
Most people are essentially two-armed, but an unlucky pilum of fire or a PeCo spell to dis-arm could still remove a persons quite permanently, yet a muto spell would have to expire.
Consider also CrVi spells that add warping points to a person, these warping points are permanent and give flaws (and eventually virtues) that are, maybe, part of the targets essential nature?
Then there are even stranger things, like Muto botches that can indeed permanently transform a person. But for something less extreme consider that muto spells can have permanent effects on their subjects as described in the sidebar on MuCo spells (corebook p. 132): "Transformations can have subtle (or sometimes dramatic) permanent effects. [...] When transformed back into a human, the target might temporarily or permanently retain some feature of the other shape."
To me it seems that the limit of essential nature is more about what hermetic magic can do quickly and reliably.
Magic seems to be perfectly capable of overriding essential nature some of the time but usually in ways that are hard to control or predict.