I think after you do a few of them they stop being stressful. I haven't been successful on every coding interview I've tried, but I've been reasonably relaxed, either succeeding or not. I would say they're not always an accurate measure. It's just like when working alone, I sometimes have a simple problem to solve, but something goes wrong and it takes much longer than it should. That is normal I'm sure. Being productive just means you're pretty good on average, not that there are no "oops" moments.

I do find that spoken, problem-solving interviews are stressful because of the need to talk about your thoughts while thinking them. I'm usually only able to think quietly.