Allo is apparently not meant to compete with iMessage. It's locked to a single handset (you lose everything if you activate on another one) so forget about it working across your devices. It doesn't work on tablets at all. It doesn't support SMS. It seems like a more limited and less convenient WhatsApp competitor.

It's basically useless to anyone that doesn't have other people willing to install yet another chat client in addition to already having Hangouts, Facebook Messenger, Skype, WhatsApp, etc on your phone.