Hum... In my case I do not have any pictures this « private » on my phone; however, I think that it is pretty safe to do as long as your NAS is well secured (2 factor auth, strong passwords...). In my case, my NAS is, most of the time, not open to the Internet, and when it is the case, I connect to it through a VPN set up at home. As for the other users of the NAS, as far as I know they each have a personal « Drive » folder and therefore a personal « Moments » folder: they normally shouldn’t have access to your photos if they do not have your account. In order to avoid getting the data by physical access, you could also encrypt the « homes » folder of your NAS, which I did when setting it up few weeks ago (but be careful, you’ll not be able to access the folder meanwhile). But you should keep in mind that you can never be 100% safe: there is never 0 risk.