A known story. Filicide, matricide, and vibes. 
There’s a reason why Elektra’s end of things isn’t that popular; it’s just not very interesting. It also didn’t help that this book didn’t focus enough on Elektra till the last third of it. I would think a book named after a character seeks to make me understand the character more, not leave me bored and abhorrent of said character (not saying I have to love every protagonist but at least let my hatred have more substance to work with).
It felt more like Clytemnestra’s story. She was layered, complex, and her POV was tons more interesting. Elektra’s fixation on Agamemnon despite knowing what he did was weird and I found myself getting tired of her continuous and frankly unnecessary whining.
I also still wonder about the point of Cassandra’s POV. It was nice but Iphigenia’s would have been better to create a more robust view of the Mycenaean women’s relationship with their patriarch. 
I did enjoy the writing. It was well done and the story made me rewatch Helen of Troy (still love that movie) but I think I’m done with mythological retellings for a while.