I bought one at St. Anthony's. It's nifty in that it actually does work and there might not be an aero penalty depending on how you mount it BUT the mounting system is terrible. It's horrible. It's unworkable. This needs to be on some sort of an adjustable/articulating mount to give you more flexibility in how and where you mount it. Surprisingly, this is less about angle and more about its relationship fore/aft of your eyes. You can actually rotate the device up/down quite a bit without having the displayed image change materially. It's pretty freaky to be honest.

Is it unsafe? I can't really say. The picture isn't great but it also isn't bad. It's certainly better than no forward visibility when your head is down. Would it encourage people to ride with their head down? Maybe, but people ride down with their head down from time to time anyways. I suppose the risk here is they become dependent on this device to look forward which could be unsafe. It's hard to judge and I've only use it on one ride so ¯\_(ツ)_/¯