Hot take: Leaving a negative review due to an update you dislike is completely valid, just like leaving a positive review due to an update you do like would be.
> sell 3 million copies in the first week
> reach a player peak of over 500,000
> receive universal praise on release
So how does Slay the Spire II currently have a "Mixed" rating on Steam?
Because Steam user reviews are not really reviews in the traditional sense. Instead, they're used as a bargaining chip to communicate real time grievances with game updates, bugs, and developer choices at the expense of a game's actual overall quality and reputation.
This is one of the many reasons why I don't trust user reviews and scores.