FAA restrictions make ultralights almost exclusively recreational craft. It's been a while since I've hade to cite the pertenant regulations (FAR part 103) but they are daytime VFR only (1/2 before sunrise and 1/2 hour after sunset), and if I remember right, there are restrictions regarding operating them over urban (highly populated) areas (minimum 1,000 feet vertical clearance).
As far as ensuring the safety of an ultralight goes, when I built mine, before I would strap my tender pink tush into them I would always test them. That means I'd assemble a copy of the wing, and then stack weight (cement bags) equal to 5x the actual max load (254 lbs (mandated maximum empty weight of the aircraft) of plane, plus 30 lbs of fuel, plus 250 - 300 lbs of my tender pink tush) the wing would have to carry: if it held that weight, I felt better about it. I'd also borrow my friend's plumbing truck, strap the thing to the pipe rack, and make runs up and down the taxiways of the (Chandler, AZ Municipal) airport to check the control surfaces for balance and controllability at operating speeds (drag racing a plumbing truck with an airplane strapped in it's cargo area, at 80 mph up and down a taxiway at 20:30 at night sure leaves an unforgettable impression on the airport manager). After that it was safe enough for taxi tests (taxiing just the airplane up and down the taxiway this time, testing the airplane's response to the engine, and breaking in the engine). Then "crow hops" along the runway. Then (and only then) was it safe enough to take off, fly the airport pattern, and land again. I'd log about 40-50 hours of flight testing the craft before I'd try anything else with it.