The ability to shut down the primary calling card of the format (commanders) without any thought or strategizing (compared to something like Nevermore, which you get locked into) just makes it undesirable
The last lines of text help it skirt Commander Tax by sending The Scarab God back to your hand every time it dies
This is not as flexible as Introduction to Annihilation or Scour from Existence, but its cheaper and got more uses
Brainstorm is perfect, or even just cards that scry so that you can try to ensure you hit something big off the top
Whether you want to attack or use the tokens to mill with Altar of Dementia, for example, is up to the player
This third ability comes in handy in several Commander strategies (filtering cards by paying life is usually a very good deal), but of course Mystic Forge is most valuable in colorless or artifact-focused builds