Special 2-seat exemptions are granted to instructors for training purposes only. These training aircraft can weigh 496 lbs and carry 10 gal. of fuel. All single seat ultralights which exceed the above limits and any 2-seater not used solely for instruction must be registered as an Amateur built aircraft and must be flown by a licensed pilot. Regulations vary outside the USA, but many nations allow more weight, speed, fuel, and 2-seat operations at the expense of more safety requirements. They have a tremendous advantage over regular aircraft due to their low weight and speed.
Minor accidents cause little damage and major accidents are less often fatal.
As with hang gliders, when they were first being invented, there were many poorly designed ultralights being flown by untrained pilots. Gliders have no engine and the operators do not consider that a safety factor. Hot air balloons can only barely control their direction. Each type of aviation activity must be conducted within its design limits. Accident statistics are difficult to evaluate.