Does Allo also require one to necessarily allow access to contacts on the device to use it? This was a must in Duo, and so I abandoned the app after denying it permission to access my contacts because it wouldn't work otherwise (like allowing me to enter a phone number manually). Regardless of what other people generally do, I wouldn't want to upload my complete contacts list to a service provider like Google (or Facebook).
Also, the default being not end-to-end encrypted is not favorable to me. But I do like the option of having it available (like Telegram's secret chat).