Because as soon as I started reading The Garud Puran, the way it describes the hell, and how Yamadoots take souls thru the hell, I could not read anymore, because that is not the way my understanding has formed after my other readings. Yes, we have to resolve our Karmas, but the way Garud Puran describes it, I was not sure those Souls will learn anything from it, may be they will learn that what I did was wrong, and I should not do it again. But that is not resolving Karma, the wronged sides totally left out. But then again, I did not read further as I didn’t agree with it, may be it mentions the wronged side later on🤨.

Puranas are not considered as supreme authorities. They belong to the class of Scriptures called Smriti. The authentic scriptures are the Vedas, Upanishads, Gita. They are regarded as Shruti. My own impression of Garuda purana is that it was possibly written with the idea of instilling fear in the minds of people, possibly under Christian and Islamic influence, to prevent them from going astray. But as you rightly surmise these things hardly help. 

As to the Katha Upanishad, it is a Shruti so carries its own authority.  Its theme is the discovery of the secret of immortality and death is only a means and occasion to reveal the secret.

Affectionately,

Alok Da