One is the basic "environmental" interference with competing radar signals from other devices. Imagine how much more complicated this gets when a majority of competing traffic is likewise equipped.
But the other risk, which seems to be mostly ignored thus far, is sabotage. Can you design a system that is hardened not only against stray competing EMR but also against attempts to hack or vandalize your system into misbehavior or malfunction? For example, consider a terrorist plot. Or even a smart neighborhood crazy who wants the neighborhood kids off his lawn and neighborhood traffic to stop during his midday nap. Or, a British organized crime gang using stopped traffic to cover an escape[0].
When all the cars on the road are self driving, I don't see why they would need to have all their radars on at once. I imagine they would be able to communicate with each other and form a complete picture of the entire roadway without all of them having their radars on (or perhaps running at a lower power?)
This would be very difficult to train with ML, however. I'm sure there are other downsides I'm not thinking of
From the "How it works" Google self driving car page: "Sensors
Lasers, radars and cameras detect objects in all directions"
It uses many kinds of sensors. Yes I know about clutter, I've spent quite a lot of time in the radar industry. But by combining data from radars of multiple wavelengths, it becomes pretty feasible. Though yes, difficult.
It strikes me as curious that someone who spent a lot of time in the radar industry would make the original statement you made, at least in that fashion. Also, radar is subject to interference as well. In addition, there are many things that act as "natural" corner reflectors that would pose more challenges, I can only think of some mountainous snowy / icey nothern US/Canadian routes and just thinking No way Light sensors and Radar sensors will conquer this.
We need to fundamentally redesign our road systems to accommodate self driving cars. That might happen... in 50-100 years...after we address the already crumbling infrastructure we have. For perfect, sunny conditions, like the roads in Nevada all these self driving startups are doing their testing, with straight flat landscapes, I am sure the tech can work fine. Rest of the country, maybe not so much.
The original comment was hasty, I admit. But there are a lot of ways to tackle these issues.
Radar clutter, attenuation, penetration, etc are all affected by wavelengths. By using multiple frequencies, you can get a better idea of what is and isn't really there. As far as interference, there are ways to filter out noise. The car presumably knows how fast it is going. Therefor, it can do doppler filtering on the received waves. IE... car knows it sent waves at 50 GHz and is traveling at 60 mph. It knows to expect a response at ~54 GHz from the front and 50 GHz from the side.
For tracking objects, you can use a pulse-doppler radar [0] to get both range and rate information.
Anyways, if this is true, it's a grim future then. Radar is incredibly trivial to interfere with.