Number1 / Number2 = %difference
PvP/PvE = % difference ---> PvP = PvE * %difference (battle fatigue)
So why hate Battle Fatigue? The problem is not necessarily evident, because once players hear there has been an overall change made, since it's hated, they rush to confirm. To do that, it seems a practical, if the overall change has supposedly only been dealt to PvP, just check how much it differs now from PvE.
The real problem emerges, if you convert the method into a general form. To confirm the statement that an overall change has been made, what the method truly does is:
Current value = Past value * %change
The formula uses 'past value', which is very much concerning, since begs for the question: How do you know what the value was in the past? It would need time travel to have a trustworthy result out of this method.
The way players use the formula, is that they simply replace the 'past' with PvE. It works, sometimes. But is very far from a reliable process.
Because PvE can change on its own, despite having similarities, for example it can increase the overall dmg in itself to infinity, simply offsetting the increase by higher boss HP. So it can cause a difference, as such, a 'Resilience' without affecting PvP at all.
Basically, the statement of having 10% Battle Fatigue/Resilience holds the information of 'These PvP spells are 10% different than their PvE versions.'
Which may leave questions such as, what Battle Fatigue/Resilience does unanswered.
It is used to keep overall good balance between HP - DPS - HPS. For that I made a calculation process example including Battle Fatigue along with other possible methods.
The example is for an overall too high HPS case, with having an outlier too low spell.