"How comfortable are the leather strap on harnesses?"
It depends. Like anything, you usually get what you pay for.
My wife has the Spareparts Joque and really, really likes it. In fact, when she first got it, she quit using her other harnesses. She had several Sportsheets nylon harnesses and a thick leather harness that was stiff and uncomfortable. The Spareparts proved to be superior to what she had previously. It is a great harness and she still uses it on occasion.
However we both love leather and after reading so many positive reviews on the Aslan leather harnesses we just knew we needed to try one. When Eden had a significant discount on the Aslan Leather Minx harness in red, her favorite color, we just could not resist. Wow! It is a beautiful harness. She loves it. I love it. We love it. The deep cherry red color is awesome, the leather is so soft and supple, the smell is intoxicating, and she loves wearing it. She has had no complaints about the fit or any discomfort wearing it.
Her earlier thick leather harness did not work so well for us., but we knew not to give up on our quest for the perfect harness. The Aslan leather harness has proven to be her favorite harness. So, if you love the look and feel of leather, I'd definitely consider one. Good luck and have fun!