For Shaer, she doesn't actually resent Shuni for originally sheltering Celene, who changed her name to Suren; that's their freedom.
And compared to others who also divided divinity from Celene but don't even look at this Maiden of the Moon under her deterrence, although Shuni has many flaws, she was at least willing to lend a hand.
Shaer, despite her grudge, absolutely does not deny that Lady Fiery Hair has some character... of course, it depends on who you're comparing her to.
If Shuni's actions were in William's place, it could probably be considered ungrateful, but in Toril, even the Sun and the Earth, who fought with her Shaer for so long over Celene, when facing a gravely wounded Celene who couldn't even use her name, acted as if it had nothing to do with them, Shuni turned out to be the most conscientious one.
