The Garleans have also had a millennia to grow past being driven north as a nation.

Nidhogg is wrong to have revenge on Ishgard because it's not right for the decedents of the slayers of Ratatoskr to be blamed for something their ancestors did a millenia ago. And Nidhogg is old enough to have been around for Ratatoskr's murder.

I don't see how it's any more right for Garlemald to get revenge for what happened to them either, when the Garleans that took over Illsabard weren't the ones who were wronged in the first place. And the people who they're subjugated certainly aren't the ones who drove them up north either.

There seems to be a parallel (intended or not) between Nidhogg's Hoard and the Garleans. Both of them are people who were wronged long ago, and both of them are getting revenge by taking out their anger on the decedents of the people who wronged them. I have a hard time seeing one set of standards being applied to one, but not the other.