All three of these are bonkers. The Samut planeswalker is probably better than Domri Rade and Sarkhan Vol in terms of effects...and it enables Gideon shenanigans in a Naya deck.
Did we really need a fifth Gruul planeswalker, though? Boros, Golgari, and Rakdos are so far behind.
Sarkhan Vol, Domri Rade, Xenagos the Reveler, Samut...is there one I'm missing? I only count four Gruul planeswalkers. Still a lot, though, and still more than any other color combination.
Arlinn Kord is the one your missing. It's only really used in G/R Energy or that Electrostatic Pummeler combo deck.