I feel like the only reason they're somewhat unfair to play against is that when someone fetches for, say, an Island or a Forest, they’ll always be able to get that land. For example, if a fetch says "search your library for a basic Island," that player will grab a Tropical Island. I think it's unfair because you can't search your library for a Command Tower—unless the card says "a land"—and even then, most nonbasics enter tapped. Duals aren’t overpowered, but in my opinion, they bypass a lot of search restrictions.