Were Momoshiki and Kinshiki legitimate threats to Kaguya's power?
According to Sasuke, Momoshiki and Kinshiki were the reason Kaguya was building a Zetsu army; that they were the threat scarier than Kaguya that she didn't want to face alone. Do you guys buy it?
Given that there's no real evidence to the contrary, and that we don't know the exact circumstances, yes.
They are nowhere near as powerful as she was after she ate the Shinju's fruit though.
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"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
Yeah. Kaguya wasn't exactly a fighter though, and while the White Zetsu were pretty weak in direct combat, strength in numbers and all that. Who knows how many Ōtsutsuki were running around that other planet when Kaguya went to the Earth though.
IMO, Sasuke might have jumped to a conclusion and picked the wrong guys, but until Kishi says otherwise, Momoshiki & Kinshiki are the threats.
__________________
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
The novel says that they're the threats, yeah. However, while they do seem much more powerful than I had initially believed, they're not peers of Kaguya whatsoever.
An interesting thing to note: Until now it's appeared that Momoshiki doesn't know any Ninjutsu himself, never learned any; he can only amplify (via pills) and fire ninjutsu he's absorbed. At the end, though, he summons this massive red Rasengan after eating some chakra pills... I don't believe he ever absorbed a Rasengan. I could be wrong, he may have absorbed one before the seen portions of the fight, but...
Don't the duo say that they're retainers of Kaguya or something?
__________________
"To all visitors from Transylvania looking for the head of Voivode Dracula: Yes, we have it. Yes, he's dead. No, you cannot see it. No, he will not return and invade you again. It has been over thirty years, please stop pestering us."
They say something along the lines of, "We came to finish what Kaguya started". However, Kaguya's scroll made it sound like if she did succeed, they'd come after her for the fruit anyway. She built the Zetsu army to prepare for their coming.