I want to say that, compared to the other PF classes, it's pretty heavy. While of course there are spells and everything else that makes a martial's life Hell, those would probably cut through you, DR or not. In that case, we have to look at things that a martial could normally face, and as some of those in here already pointed out, it negates a *heavy* amount of damage. A CR20 creature on average probably deals, what, 120 damage? 90?(According to the PF Monster Creation). Let's say you have ~180HP; d10 class, Con mod of +3, going for average, here. That's 2 hits on average, which gives you an effective boost of 40HP.

Lesser creatures are going to have an even harder time piercing through, with a CR19 creature, with a low attack of 82 damage, giving you sixty effective HP. And those numbers just keep going up for how many hits it takes. Going up against something like a Solar gives you effective HP probably equal to double, even triple your normal HP.

And that's just a race and (apparently?) a feat. Not even a class feature. Compared to everyone else, it's... pretty strong. Strong enough to search for reasons not to have that race for any martial, with "that class's DR won't matter" being not much against it.