ARGH definitely they can be too bright. There's a long cycle path through the middle of Hyde park in London which can be pretty dark on a wet winters night. Ordinary LED lights are perfectly visible in those conditions, but someone would always come along with some kind of nuclear fusion powered gigawatt arc-light strobing away on the front of their bike. When the light is on it hurts your eyes and when it's off you're still blinded and can't see where it came from, making it very difficult to know where they are, like some kind of nightmarish quantum physics conundrum. An electron in a cloud chamber. You just have to stop and close your eyes and wait for them to pass.

Please. Dont. Do. That.