Playing sideboard cards that are only good against Burn is likely a losing battle, no matter how good Kor Firewalker or Dragon's Claw might be in the Burn matchup
The fact that it can also hit artifacts is incredibly relevant, too
The biggest downside to this card is it has to die, so if someone uses exile removal on it or it gets blinked, you do lose those cards permanently
Red-Green is all about playing big, stompy creatures and other expensive spells, with extra payoffs for casting anything with a mana cost of four or higher
The only fix I can think of is making your own lords, and making sure those lords are playable on their own
If you'll notice, I included the rulings text straight from the MTG website