Partial may seem like it fails most of the time based off of this forum because everyone currently active and posting are all alive having survived our attempts and writing about our failed attempts, often in a defeated and hopeless manner. This forum is littered with failed attempts because the ones who succeeded simply cannot post. The main reason for people to fail with hanging, partial or full, is because they didn't plan diligently enough to know they can be alone and unfound and the other reason is they simply backed out. No one is suddenly waking up while hanging from a rope while they're still suspended and their arteries are crushed. It only takes around 11lbs to compress the carotids, so unless their anchor broke it's a pretty reliable method. Much research you can find online notes hanging, partial or full, to have an average 70% mortality rate and I assume the 30% fail rate is down to people being found, backing out, anchor breaking or other less common or rare reasons. Also, according to different case studies and research, it seems that partial hanging is more common than full suspension "Partial hanging was present in 68% and complete hanging in 32%. The most common reason for attempting suicide was under the influence of alcohol (31%) source: nih.gov".
Partial only means the body is making contact with the ground in any fashion. Whether it's feet, legs, knees, buttocks, or some people are even found laying down.
I've never experienced bruising but I guess it depends on the ligature (what it's made from and the size). If you try to immediately hang downwards then you're more likely to experience bruising than if you were to lean forward