Theres counter distribution, proliferation, and reanimation, and even creature lands have their own payoffs
Catching you up against fast opponents or going over the top in the late game are both common functions of this creature, and it enters unfair territory when you scam it with blink or Supernatural Stamina effects
Creatures you begin to control later in the turn wont get +1/+0 or gain menace
Even without casting The Ur-Dragon, it remains one of the most intimidating commanders in the game
Invasion of Mercadia C-Tier: Basically the battle version of Bitter Reunion, except the ability to give all of your creatures haste is likely more of an upside than potentially flipping into a 3/3, which means this is probably just a worse version of a card that already exists
(5/3/2019) Karn's first ability affects only artifacts on the battlefield