The problem is twofold:
First, mythic divines are meant to be overpowered.
If “balance” is a 5, that’s about where a legendary divine should be, and a mythic is more like a 6. They’re meant to be more “Oh $@#&!” than “Yep, that’s a dragon all right.”
Second, the power level of the dragons is highly dependent on level and - thanks to the stupid spell scaling - gear/runes/etc.
Which meant that while the people at the very top experienced Pathox jumping to a 7 or 8, most players…didn’t. And then PG decided to apply an across-the-board-nerf to the dragon itself, rather than to the mechanics that were causing a problem (i.e. gear). This had the signal effect of making the players at the very top STILL get a boost, just not as big of one - say, to 6.5 - but ALSO meaning that those not at the top got no boost, or even a noticeable nerf.
Basically, people are pissed because rather than fixing a problem with the stupid design of spell scaling, PG decided to nerf a dragon that people worked hard/spent heavily to get. Which didn’t properly resolve the power issues at the top tier, did cause Pathox to get weaker relative to other dragons, AND, for some, to also get weaker relative to bases.