Honestly, the worst part of Jean Grey’s resurrection is the treatment of Madelyne Pryor and Rachel Summers after her return.

Agreed. 

While Inferno has its charms, it did do Madelyne Pryor dirty, turning someone who had been the Australia X-Men’s “guy in the chair” into a crazy vengeful ex.

But the biggest missed opportunity was Rachel. I would argue that Claremont didn’t really know what to do with Rachel – hence the really abrupt twist of her trying to kill Selene (one of his worst villains, imo) to Wolverine trying to kill her to teach her that killing is wrong (???), to her going into the Mojoverse – until he started writing Excalibur.