Speaking as someone playing a
githyanki pryromancer at the moment...
All it's really done is made my character actually effective against the couple of fire-resistant critters I've bumped up against. I'm a pyromancer. All of my attacks (though by choice) are fire damage. If I don't have this feature, I can't contribute very much at all against fire-resistant creatures. And if the creatures happen to be a theme in the campaign....I'm back into 3e Rouge vs. Undead levels of unhelpful.
Me and the 40 Legion Devils I killed single-handedly in the last session speak to the fact that this works well, since otherwise, the party's only controller would be sitting around doing jack squat while the army mowed down our melee troops.
Izzussan is geared toward damage, too. I'm in no danger of outdoing the party strikers (an Avenger and a Warlock) any time soon, though I'm better with crowds.
Fire immunity still works, though it's rarer to find that on monsters than resist (and I'll probably pick up Burn Everything sooner or later). 4e cuts down on immunities in general.
Basically, it's not overpowered, it's there to help the pryomancer do his job without borking him depending on DM monster selection.