Merc was as mindless as Acno was. It was stated by Ignia that he was relying on Instinct the same as Acno. Yet he was able to take a continuous strike from all Team Natsu who was enhanced by Wendy while he was only half powered yet Acno swatted by a rain strike from Erza who was not enhanced and all by herself while Acno was enhanced. EoS Erza was clearly superior due to be doped on PoF as she was capable of doing that so there's no point in downplaying Irene's dura against a PoF empowered Erza hyped on PoF Wendy juice.
The same can be said for Zeref who had to fight a full on PoF Natsu. He gets downplayed a lot despite taking strikes from a Ignia empowered Natsu in a weaker form.
PoF > Everything in the last parts of the manga. Currently no one has used PoF in their fights to break their limits so dunno how much PoF will be able to work in 100YQ, but you shouldn't downplay other characters who lost to PoF empowered characters. Hopefully it won't be as bad as Alvarez, but I won't surprised if they pull that bs against Aldron as they have no actual way to fight him.