Obviously Nagato and Madara have used this ability to absorb numerous ninjutsu attacks, but from Nagato's application of it during his Edotensei battle with Naruto and Bee we can see that it can also be used to absorb plainly chakra. This leads me to think that the ability is entirely just absorbing chakra, and this makes me consider that perhaps the Rinnegan cannot absorb all ninjutsu, but merely all ninjutsu that consists of solely chakra?
As an example, consider someone was battling a Rinnegan user and used the Suiryuudan no Jutsu when there is no water around, creating the water dragon bullet purely from his own chakra; I think there's no doubting that the Rinnegan user would easily be able to absorb the entirety of this attack. But if that person had used the Suiryuudan no Jutsu when there was a substantial water supply around only using chakra to change the water's form, would a Rinnegan user be able to absorb that jutsu, even though the bulk of the jutsu is made of natural substance and not chakra at all? Or perhaps they would be able to absorb only the chakra-part of the jutsu that contains its form, and consequently make the jutsu less lethal? If they could absorb the substance, then why do they not simply apply 'absorbing substance' as an attack?
It's perhaps a long shot, but it's an idea I've been interested in lately, what are your thoughts?