Dynamic Soaring is quite impressive. At the world-record speeds they were having problems because the radar guns they were using (the type police use) were not picking up the speeds anymore. I think there were only a couple of particularly good ones (particular specimens not particular models) that were still able to register something and not easily.

Initially they had issues with planes blowing up mid-air because they were using hollow-molded planes and as soon as a bit of air got into the seam at the wing leading-edge it would open the wing in half quite spectacularly. People tend to stand next to a large rock on the mountain in case they need some good cover. The DS world-records were done with some custom built, very large very heavy gliders. There's quite a bit of great engineering in the R/C hobby.