It's just right now, because CSS doesn't support it, that SASS code gets compiled into large selectors if you aren't careful.
It makes deep nested code hard or impossible to reuse. It tightly couples style and markup, so a change in the latter can break the former, etc.
So while it is a nice feature to have, I'd recommend using it scarcely and with great care.
Any nested SCSS is impossible to reuse outside of its parent scope. If someone is using nested SCSS and expecting to reuse that code elsewhere then they are doing it wrong. It's the same problem that exists if one were to create a class as a child of an ID and then expect to use that class outside of the ID's scope.
You can edit minified code if you've had enough coffee!