Warning beforehand: This is one of the single most explicit mangas on this site that isn't tagged as hentai. The female protagonist of this series spends more of it naked than clothed. Some chapters are just pure sex with both participants fully nude and all their parts revealed (censor bars get thrown in in those cases, or maybe some object from the foreground blocks the view). Borderline H is barely sufficient to describe this. 'Smut' doesn't even come close to covering it.
Best way to describe this manga: An unconventional alien invasion story using somewhat unique mechanics to tell an interesting story
Worst way to describe this manga: A rape-filled wish-fulfillment fantasy comic for masochists of both sexes who are into science fiction to use for masturbation purposes
The reality is somewhere in the middle of those two opposing descriptions. I honestly don't even know how to rate this manga. I mean if the latter statement above is its intended purpose, it's a 10/10. It's literally perfect for that role. As a story, it's 'fine' or 'good' at best, but the sex parts sort of interfere with that task. There's some tropes I really enjoy in this story, and that's probably the only reason I tolerate it so well. I really like stories involving hidden identities, heroes being forced into the hero role, secret wars, and there's one other aspect involving leadership that comes into the picture later that I find especially interesting (don't want to spoil here). After all that talk, I'm probably going to mark it as a 7 out of 10.
As a side note, it probably needs a sexual violence tag considering the protagonist is sexually violated in almost every chapter. She even refers to it as 'rape' herself.