Potentially very dangerous to people and equipment nearby.
It is possible for high-speed dives or swoops to reach very fast speeds, certainly enough to injure someone if they are struck directly, suffer severe burns from the lines sliding across their skin. Spectators should not be underneath kites nor within a crash zone around them.
Anyone working with the kites, especially anyone handling the lines, should understand the dangers of fingers/hands/heads getting wrapped in lines and treat the kite lines with proper care. People have lost body parts and even died from those mistakes.
Equipment can be struck at high speeds, tangled, or otherwise damged, but generally equipment can be replaced.
Kites should be kept under control at all times. Large or small, kites need constant supervision to adjust line tension as needed and ensure people stay out of potential harm's way.