Is Radar useless? Well, it’s inconsistent. In Monsters vs. Aliens, the villain Gallaxhar's honking massive starship seemingly just appears over Earth without anyone noticing it coming. By contrast, the movie opens with a relatively small meteor being easily detected and its point of impact accurately pinpointed by one small remote station.
The A-Team movie severely abuses radar through the film: An air battle takes place between the A-Team in a stolen C-130 and two Reaper drones (which is impossible to begin with but that's beside the point), and Lynch sends an AC-130 gunship to attempt to kill the A-Team, yet all of this takes place in German airspace with no apparent reaction from the Bundeswehr.
In Stargate SG-1, smaller Goa'uld ships often have cloaking devices, but on several occasions alien capital ships appear over Earth and go completely unnoticed by anybody not related to the Stargate program. In some instances, however, they seem to go unnoticed at first but it turns out in a later episode that outsiders did notice: the Russians are clued into the program's existence when they recover wreckage of Thor's flagship Beliskner from the bottom of the Pacific following one of their submarines, whose radar did detect alien spacecraft, going missing in a related incident.
In the movie Independence Day, an alien mothership nearly as big as the Moon isn't noticed by deep space radar until, in astronomical units, it is within spitting distance of Earth.
After the U.S. attack on the alien ship hovering over Los Angeles, a large number of alien fighters are able to make a surprise attack on the U.S. Marine airbase at El Toro even though the base should have been able to see them approaching with its radar.
In the movie Armageddon, an asteroid the size of Texas somehow goes undetected until it is less than two weeks away from striking Earth, due to debris destroying a Space Shuttle and bombarding New York, and even then an amateur astronomer has to show NASA where to look.
Astronomers started hunting for space objects large enough to pose a threat several years before this film came out, and while they have missed incoming meteorites since (the 2013 Chelyabinsk meteor went completely unnoticed until it hit the atmosphere because it came from an angle close to the Sun), the stated size of the rock in Armageddon would make it bigger than Ceres (which was detected with an optical telescope all the way back in 1801) and therefore almost unmissable.