Let's get one thing out the way first--this card is just terrible compared to Key of Ys. Granted. But most cards in the game are going to be terrible compared to Key of Ys, so let's not make that a serious factor in our evaluation of this card. +1 to all stats is an extremely powerful effect, even if Key of Ys might make it seem paltry in comparison.
Still, for a lot of Mystics builds, you don't really care about all of your stats. Plenty of Mystics never use Combat and rarely use Agility. You might not even care about anything except Willpower, to be honest. Which means this card is just much, much worse than Holy Rosary or St. Hubert's Key.
If this card has a niche, it's surely in solo play. There a Mystic is most likely to need all stats, and the card's drawback of sealing the Elder Sign is the least painful.
But in multiplayer, I just see very little reason to play this card instead of those other Amulet options. Even if this card were 0 XP, I would still prefer the Rosary and/or St. Hubert's Key. Since it's 3 XP, it's not remotely attractive.
Finally, there's the social element to be considered here. If you're playing multiplayer, you're taking away quite a bit of fun from other players when you seal the Elder Sign token. Not only will your teammates perform a little worse, they will also lose some of the most exciting and dramatic moments in the game.