It was designed to be the minion part of a DK deck build but because it’s a new class there isn’t enough to build around a multi rune deck so straight runes will be the most powerful.
The path of Arthas should have been twice the size of an expansion for DK that it was. Even at rotation DK will still be behind two sets worth of cards. Making it double the issue is that DK is the kind of class they designed that needs MORE cards in its pool than other traditional classes to make it interesting.
IF they give Unholy effects from other runes that don’t make sense it will really mess up the balance. I don’t really see DK becoming a real class outside of pocket meta’s until the 2024 rotation.