Casting it is like Teferi's Protection, shielding the player from everything until their next turn to represent a Ring wearer turning invisible while wearing it
Adding extra restrictions and benefits like the ones brought by conspiracies can make for some super interesting decks
Though it hasnt been explained in detail by Wizards of the Coast, it seems fairly obvious that their licensing deal with Marvel/Disney only covers them for printing a set physically, and not using the comic book characters and settings digitally
On the plus side you draw a card whenever a land enters the graveyard from anywhere
Adding flash does help set the Iceberg apart, but so does its waterbend ability
Fellwar Stone Ain't no such thing as too much mana, and Commander rewards playing cards that scale with the number of players at a table, making Fellwar Stone a welcome addition to power-hungry, multi-color decks