When comparing WhatsApp vs Viber, the Slant community recommends Viber for most people. In the question“What is the best team chat software?” Viber is ranked 38th while WhatsApp is ranked 54th. The most important reason people chose Viber is:
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.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Straightforward interface
The interface is very simple, fast and easy to use. You can start using it as soon as the app has been downloaded - it does not require setting anything up or registering. You can communicate with anyone in your contacts list that also uses WhatsApp. Includes useful features like seeing when a contact is currently available or typing.
Pro Widely used
WhatsApp reported 1.5 billion users in 2018. Your friends are most likely already using WhatsApp, so it's easy to get setup.
Pro Effortlessly builds your contacts by using your phone number as identification
WhatsApp will automatically find all of your contacts currently using Whatsapp and add them to your contacts. This makes it really easy to setup and given that it's the most popular messaging app it's likely that a lot of your contacts are already using it.
Pro Message mirroring on web app
WhatsApp can be used from the desktop via Chrome, Firefox and Opera browsers. Accessing it requires using the Android app to scan the QR code that's presented on web.whatsapp.com.
Pro Signal protocol
Whatsapp uses the state-of-the-art Signal protocol that ensure strong end-to-end encryption.
Pro One-on-one texts can be encrypted
WhatsApp has implemented end to end encryption for messages sent on it's service. Even WhatsApp has no way to read a users messages.
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.
Cons
Con Shares meta data with Facebook
WhatsApp shares your messaging meta data with Facebook if you do not opt-out in the settings. If you use the same phone number for WhatsApp and Facebook, the profiles can easily be linked.
See https://www.whatsapp.com/faq/en/general/26000016
Con Tied to carrier phone number
WhatsApp cannot be used by people without a smart phone with an active cell plan. If your phone is lost or damaged, or if your phone number changes, you cannot get messages sent by friends without them updating your phone number. You cannot send message to them until you are able to get SMS messages at your phone number.

Con Proprietary
Not open source.
Con Hard to migrate history cross platform
Migrating your chat history from Android to iOS or the other way around is not so simple. In some cases it requires a 3rd party app and in some the only way is to migrate specific chats one by one.
Con Web interface uses message mirroring
To use your computer to send messages your phone must be on with a consistent data connection.
Con Single device
You cannot use the same WhatsApp account on multiple devices (e.g. a tablet, a PC and a cellphone). The account is tied to the device with the specific cellphone number. Chat history is deleted when you sign in to a new account, unless you have created a backup.
Con No cloud sync
Messages are not stored on WhatsApp server for privacy and security reasons.
Con Limitation on shareable file sizes
WhatsApp now allows users to share files of any type, but there is still a 16 MB size limit.
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.
