They do slip. That's what I used on the school horses. My felt one holds. Okie has never gotten rubbed or lost hair. I did have the change from the rope to nylon halter though. He was getting rubbed on his face by one of the knots.
I've never used a smooth leather cinch, but I think they would be very comfortable too.
I used a neoprene pad, when I had the trouble. The problem is what you said. Heat builds up under them and then they stick to the horse and it just pulls.
I had one and did a competiton in it with Belle. It scalded her so bad that she had blisters on either side of her withers. When the hair grew back it stayed white. I was going to pull her and not go out the second day, but some lady brought me a pad she designed, it had surgical fleece on the underside, but the padding inside was astro turf. She said try it to the first PR stop. When we first started I really felt like a heal, Belle seemed so stiff and sore. But after a few miles she was moving out and seemed to just get better and better, so I finished. When we got back I was really dreading pulling that pad off, I figured it would pull skin and everything off on those spots. But when I did, they actually were flat and smooth and looked better.
I bought one of those pads from her and used it for about 4 years. Now I have a wool pad and it's been real good. You just can't skimp on pads or cinches. They need to be well made out of good material.
Becky