Here a few cards that were thought of very highly at spoiler season and became next to nothing:
Past in Flames:
Past in Flames is an interesting case in that I don't think it's fair to call previous evaluations of the card wrong
You highlight an interesting point here that most card evaluations are based on the current metagame or even more specifically an individuals viewpoint of the current metagame via whatever decks they are currently playing.
This is why cards that were previously so powerful that they needed to be restricted can then be unrestricted and have 0 effect on the metagame.
Its also why some of these viewpoints of initially spoiled cards can look bad years later. For instance, if we look at my quoted comments on Dack Fayden:
vaughnbros: "I'm not sure how playable he is main deck as his second ability and ult are rather >narrow while his +1 isn't back breaking."
vaughnbros: "You mean like how hes an overpriced Careful study against any creature heavy >deck like BUG fish or Merfolk?"
BUG fish and Merfolk aren't even decks anymore. Cards that paired well with Dack Fayden: Young Pyromancer and Delve draw spells had yet to be printed. The card made a meteoric rise thanks to a lot of shifts in the metagame.