Any part failure is bad at track speeds. Few are worse than an entire wheel bearing failing. Without the benefit of the tethers most race cars are equipped with, there’s a risk of the wheel simply flying off on its own chosen trajectory with nothing to stop it but what it can find to hit.
Jeremy Rohan, a driver who was racing in a NASA event at the Eagles Canyon Raceway in Texas, found out what a worst-case bearing failure can look like the hard way. His V-6 swapped RX-8 had a bearing issue on the Saturday before a race, so he and his BSP Motorsports team went to a local auto parts store to find a replacement. He finally found a store-brand component and installed it before the race, and after a few laps it suffered a catastrophic failure at full speed on the start/finish straight.
The scale of what had happened was immediately obvious to Jeremy, who was able to actually see the tire fly by as it happened. As the on-board video shows, it seemed to break out of nowhere, popping off without any of the vibrations that signaled a less catastrophic failure the day before. With an entire contact patch suddenly gone, his attention turned to the actual process of slowing the car. For the full story and video, check out this article from Road & Track.