To me it's more about how busted the deck is. If the deck has a 14% metagame share with other decks having to pack 7-10 dedicated slots to play against it, it's clearly warping the metagame. Dredge is the deck we kinda get used to waste 6 sideboard slots. Pre-ban the sideboard had to be 12-14 slots for dredge and artifact.
About the 'island' comment, you should compare island to wastes. When we see a land with 'tap to add UUU. Use this mana only to cast instant or sorcery spells' it will make sense. Mishra's workshop has made a lot of cards get restricted so that it doesn't need to, and that's a pretty loud fact.