Yeah but rasengan isn't even naruto's strongest technique. That goes to show that without his chakra absorbtion, momo has very low durability. Indeed this does make him appear weak compared to other villains.
Also a man has no capacity to react to a bullet, whether shot from a child or an adult. Momo on the otherhand could have done something to the rasengan, but didn't or rather failed to do so. The example you should give is, give a 3 year old a knife and asking the 3 year old to stab a police officer while said officer is facing the child.
That is some serious underestimating of that Rasengan. Sure, the ending was generic, cliché, and slightly distasteful.
But at the same time, that does not even begin to discredit Momoshiki. That Rasengan was definitely far more powerful
than the majority of Naruto's techniques.
Momoshiki did fight back, and he used his super TBB like jutsu that has been shown to be incredibly powerful, far more
so than any bijuu dama ever used (note: juubi dama is different), and the rasengan not only overcame that but obliterated
the Shinju, (even if it's not the original it's still impressive) even though the rasengan's power was focused on Momoshiki,
and the rasengan still kept on going.
How dying to something like that means you have low durability I don't understand. Even Juubi Madara, after tanking
a Lava FRS (albeit he had already been weakened), was highly injured and that Lava FRS was shown to do a lot less
damage than the final rasengan. There's nothing to suggest that the likes of Juubito and Juubi Jin Madara, should they
tank the jutsu head on, would survive it. Heck, there's nothing to even suggest that Naruto or Sasuke could tank it
either assuming they let it hit them head on. In fact, all the evidence lies with any of them being obliterated.
I know they can defend, but just raw durability wise, Momoshiki ain't lacking, even compared to these other top tiers.