UFOs have not advanced technically for decades at least. In some cases, they have stayed the same for milleniums. Take the classic saucer or cigar-shaped UFOs. They have been seen and recorded since ancient times. It could be that the civilizations that use these configured crafts haven't found better designs.
The propulsion systems have stayed basically the same for centuries. For flying through electromagentic fields they use field displacement. A circulating motive field is projected from the craft to determine the direction and speed of the craft. A repulsive levitation field can allow the craft to stay suspended above the ground and also further protects the craft as it glides soundlessly through the air at hypersonic speed.
Either an internal gravitational field or inertia dampers prevent the occupants from suffering from extreme G-force as the craft changes direction when the motive field changes the direction of circulation. Work being done in Europe to suspend objects and creatures in electromagnetic fields will help us one day achieve neutral G-force inside crafts.
The engines and the power systems for UFOs are nuclear. We could learn some things from their technology. Take for instance the Roswell crash. Since the material for the craft was a reactive material that can be manipulated by a powerful nuclear energy source, we know that nuclear radiation isn't that much of a problem. The only times people have been harmed by exposure to UFOs is when they are exposed to the energy being expelled by the engines or field displacement system. When the lightning bolt was attracted to the craft's fields and blew out the reactor, there was no massive explosion that would have vaporized objects and people for miles around. It could be the aliens have perfected a fail-safe nuclear energy system that is discharged during an accident. The craft's material went from being reactive to just an ultra-stressed crystalline molecular solid material.
The nuclear fuel could have been synthetic. If it were a massive transuranic element, it could have been the type that needs either a particle field or beam to stimulate the emission of particles from the fuel. When the field or beam are shut off, the fuel becomes a stable material that is safe enough to handle unprotected since there is no problem with radiation. We should someday have suce elements once we perfect hyperlight physics.
UFOs use tractor beams to seize objects and bring them into the crafts. A tractor beam is similar to a transporter beam in that they both envelope the object being transported with an energy field. Transporters accelerate solid objects almost to the speed of light and sort of broadcast the objects to their destinations. Tractor beams use a linear induction and electromagnetic field attraction system to draw objects the UFOs. A combination of the two systems allows aliens to pass through solid walls. It could be done by adjusting the molecular frequency of the materials to match the field frequency so that aliens can pass through the materials. Even lead has a frequency that can be adjusted since radio frequencies can pass through it.
UFOs are silent for various reasons depending on the situations. Since field displacement is used, the only sound you might hear would be either the hum of the fields or the crackle of the ionized air. When the air is ionized, in conjunction with the electromagnetic fields of the earth, the displacement fields can allow the craft to travel at hyperlight speed without causing a sonic boom since the ionized air and the fields barely move if at all. A field linear induction system is used. If the craft shifts from another dimension into ours, sound might be shifted back to the original dimension.
UFOs have been able to become invisible both visually and on radar. This is due to field manipulation. It can absorb radar beams and not allow them to be reflected. If the vibrational frequency of the field is rapid enough, it can become invisible in a split second. The fields can also refract light so that the craft will appear to be invisible. The only thing that can't be hid is the mass of the craft. Since objects in motion displace air in the atmosphere, this motion can be the wake that will allow detectors to track "invisible" UFOs.
UFOs, are basically "dead end" technology. Wormholes, long-distance teleportation, "solid energy" crafts, and some other exotic means of transportation will eventually replace UFOs. They aren't thousands of years beyond our technology. At best, they could be a couple centuries ahead of us depending on when we can use hyperlight physics. Once that happens, since man can develop technologies very quickly if he chooses, UFO technology may last a few centuries before being replaced by the more advanced systems.