One of the small number of PPR issues that I mentioned was a refusal to land while in the air after I had obtained PPR by phone. Before I took off I made sure I phoned ahead to get PPR but when I was approaching the airfield, some 40 minutes flight time later, the tower suddenly had no record of my call and refused the landing. As you can imagine this was slightly annoying, you can't really argue/convey your point using RT in the same way you can by using a phone.
This was probably just a one-off issue but it gave me a flavour of what a refusal (for whatever reason) of PPR using the radio would be like and I can honestly say that I would prefer a refusal by phone any day of the week.