In all fairness to my dislike for Gaspard Noé, I probably liked this one because I misunderstood it.
The ineffable Béatrice Dalle plays the director of a film about witches starring Charlotte Gainsbourg. The first act of Lux Æterna is a dialog between these Queens where they discuss witch moments in the past that affected them. That's when I understood Noé is telling us modern day actresses are treated now like witches were historically.
I won't go into spoilers but I will say there's a male DOP at the end of both this film and and the film in the film who would've fit in well with the inquisition. (Is this character meant to represent Noé? Could it be a confession of his own misogyny?)
If the film really is what I understood it to be, then it's an excellent condemnation of the treatment of women in the film industry. If I got all this wrong, then Lux Æterna is just another shiny bauble in Noé's bag of tricks.