Whenever I meet a rider who has flush-mounted turn signals, I ask them to turn them on for me. In the daytime, they always look dim and unsafe. I was in traffic yesterday when an Audi A6 wanted to pull in my lane in front of me. Being a courteous rider, plus he signaled and outweighed me by a ton or more, I let him in.
Even though I was by his passenger door, I saw his turn signal. The reason I saw his signal was because it was a bright, flush mount set on the side of the front fender. If I wanted to install a flush mount on my bike, I would be visiting a wrecking yard to get a set of flush mounts out of the Audi. I don't know what year it was, or what other Audi models have them, or if people still go to wrecking yards anymore, but it looked good and was bright.