I've tested Viber vs Skype continually and have found that they both pretty much have the same video compression quality for video calls and what not. Although Viber isn't as complex as Skype, it does have its benefits because its video transmission protocol is a lot less complex than Skype's thus making it easier to work with your smartphone device.
Take into account that a good WiFi connection will make a big difference as well. I suggest you have no less than 5 MBs downstream and 1 MBs upstream to fully experience Viber's ability to record, render and parse amazing video data.