Cool, again, one of the users is long gone, and the other can't do it at will, and as far as we know no one else knows it. And yes I'm sure you can come up with some fanfic theory of someone possibly being an uzumaki cause they say a word Karin does, or senses like her, or has kushinas voice! Or some dumb sh t like that. You can't underrate the nuke you can't use
Btw, you can't call something underrated if no one ever underrates them, everyone knows what chakra chains can do, it's just that again... no one alive can use them at will, and there's no guarantee anyone past Karin will be able to at all