Two years ago today, Google released its 'Duo' video chat app for iPhone and Android. It was designed to replace the video chat component of Hangouts, just as Allo was supposed to supplant Hangouts' messaging functionality. Fast forward to the present, and Duo has definitely achieved its goal of making cross-platform video chat easier, even if it's not the runaway success Google probably hoped for.
Duo has continued to receive updates, while Allo development is in a holding pattern. While there are countless video chat applications available for Android, Duo has remained somewhat popular thanks to its ability to work well on poor network connections. The app is also pre-installed on most Android phones.