There's a lot of PWS who experience difficulty with specific sounds or words (like their names for example). I don't think any sound is inherently "harder" to pronounce than any other sound. I think it's always about the words/sounds and our fear of those words/sounds. Now why do I think that? When I was still stuttering severely, I once took an improv workshop. I felt like challenging myself and had a great time despite feeling hindered by stuttering on most of the exercises. One that I was able to do flawlessly however was a game called "Gibberish". Basically, two people will talk to each other in a made up language, just making random sounds. I didn't stutter a single word during that game because I was just speaking spontaneously and without thinking or fearing any words (because there were no words to think/fear). Every PWS I've seen do this exercise has been able to do it without any hint of stuttering. It's never about specific sounds – it's our thinking and fearing that makes us stutter.