When comparing Viber vs Google Hangouts, the Slant community recommends Google Hangouts for most people. In the question“What are the best Skype alternatives?” Google Hangouts is ranked 9th while Viber is ranked 16th. The most important reason people chose Google Hangouts is:
Hangouts offers both an Android and iOS app so users can participate from nearly any mobile device in addition to his Desktop.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Desktop apps allow texting, calling and enables transferring calls between devices
Viber has native apps for both Windows and Mac that allow you to send and receive messages from your desktop. You can also use the desktop to make and receive calls. Synchronizes between all devices you are using Viber on. Allows transferring calls between devices.
Pro Supports voice calls
Viber has really well implemented voice tech. It has comparable quality to Skype and you can even tranfer calls from your phone to your desktop seamlessly.
Pro Completely free
Viber uses data connection and the app is free.
Pro Clean interface
The interface is simple, functional and intuitive.
Pro See who of your contacts is using Viber quickly
Viber is tied to your phone number so it can quickly see who else in your contacts list is using the application.
Pro Android Wear support
Viber includes Android Wear support on Android 4.3 or higher and Viber 5.3 or higher.
Pro Same protocol as used in Signal
Viber's protocol uses the same concepts of the "double ratchet" protocol used in Open Whisper Systems Signal application.
Pro Public chats
It's possible to be a fly on the wall on other group or one-to-one chats that have been opened up to the public. These public chats are often promotional chats with celebrities.
Pro Supports video calling over WiFi and data.
Can call friends using voice or video options.
Pro Combines cellular and data messaging
This has an advantage in that if you text someone who uses Viber it will be free, if not it will just revert to SMS. This means you can replace your standard messaging app with Viber as well.
Pro Keys only on device
Starting with Viber 6.0 the encryption keys are stored only on the clients themselves, not even Viber itself has access to them.
Pro No need to create an account or add contacts
One of the best things about Viber is how it handles setup and sending messages. Viber uses your existing number as your ID, so you don't need to create a separate Viber account.
Pro Supports 200 person group chat
Viber allows creating groups of up to 200 people on the Android version of Viber. It also has a cool "smart notifications" feature to prevent being overwhelmed with notifications if a lot of people are active in a chat group.
Pro Insight and well documented encryption
Viber has made a documentation available, explaining in great details their encryption system and giving good insight on how it works.
Pro Mobile support
Hangouts offers both an Android and iOS app so users can participate from nearly any mobile device in addition to his Desktop.
Pro Screen sharing
Any user can share his screen (or only a specific desktop or window) while still continuing an audio conversation.
Pro Free group video support
Hangouts works with up to 10 participants using video out of the box or up to 15 if you are a business, government or school. It determines who is the primary speaker at any given time, and the camera view automatically switches to that user.
Pro Useful for businesses
Hangouts can scale to become a videoconferencing solution for an entire company, since the software can run on nearly all platforms. In fact, Google itself uses Hangouts as its meeting room video conference software.
Pro Easy to start using
Hangouts is integrated with Gmail & Google+, does not require you to download desktop software, and all chats can take place directly in your browser, significantly reducing the barrier to entry for a new user.
Pro Completely free
Google offers all features of Hangouts completely free. There are business, education and government solutions that increase the participant count to for video calls to 15 and offers support.
Pro Cross platform
Hangouts work on Android, iOS, Chrome OS, web and as a Chrome extension.
Pro Live broadcasting and instant recording
With Google+ “Hangouts on Air”, you can broadcast any hangout to an unlimited audience via your Goolge+ Profile or simply a generic YouTube link. Moreover, any Hangout on Air (public or private) is automatically saved as YouTube video that you can keep for private reference or use in any way that you would normally share a YouTube video. This allows you to record conversations without additional obtrusive software.
Pro Google Voice integration
Allows for a fixed phone number, receiving and making calls. Calling US or Canada is free.
Pro Integrates with Google Maps for sharing locations
When chatting with people while trying to plan a meet up, the share map button can be extremely useful.
Pro Android app does both sms and hangouts messaging
Cons
Con The company evades U. S. tax
The company uses the common practice of tax haven addresses in order to avoid paying US taxes despite being registered in Delaware.
Con Proprietary Software
Not Open Source.
Con No web client
You cannot login from the web to send and receive messages.
Con Constantly on
Need apps like Greenify to stop it from being constantly on and active, even if you don't use it.
Con Constant reminders of messages
If you don't check your messages (say your in a group chat and simply don't check all of them), it will flood your notifications.
Con Doesn't sync well
Sync awfully between Phone and PC clients.
Con Poor video call quality
Only goes up to 720p30 + low bitrate.
Con Owned by Google
Con Lacking clients for older versions
Con High hardware usage
Con Not recommended for long sessions

Con Won't work on Windows 8-style UI
As Hangouts uses a browser plugin on the desktop and it doesn't work with Windows 8-style UI, it has to be used in desktop mode.
