I know this is an old thread but copper nuts are used on the higher power classes in Europe with no issues at all. I have been testing them all summer on my KZ Chassis with a Modena engine that puts around 50 hp and even on these asphalt tracks with continuous abuse due to the aggressive driving a shifter kart requires on small tracks I have had no issue at all. They even offer advantages on the nylon ones, they are easier to install, faster to removed, they are flanged so don't come off and don't require the use of any kind of washer, they are also lighter and won't damage the bolt threads, and their walls are narrower which means with some wheels are actually the only type you can fit because of the space around the nuts. No corrosion either. The hole is oval which causes them to lock and not getting loose. Read this quote below:
"CIK copper wheel nut, these are the wheel nuts we choose to use. The hole is an oval so that's how it becomes a locking nut. The softer copper will not strip out the wheel studs, the only downside to copper nuts is they will loose their locking ability sooner because they are a softer alloy."