I wasn't comparing the new phones with the HD2, only responding to the statement that "Viber has always been an issue on Android". You mentioned issues on a newer device like the S4, but I know someone with an S4 that it's fine for.
Yes indeed i have , the reason for this conclusion is based upon the EU S4 version that is sold in our country. Its either equipped with Telfort Mobile,Vodafone NL or KPN.
These network providers delivers the device with a vast amount of adware applications which creates a very unstable performance. Some providers even deliver the devices with subscription running on a for the provider developed ROM.
I have noticed that the phone loses at least 2,5GB on memory new out of the box. This can the rom its self ,considering that the S4 has about 18 motion sensors and the HD2 only 5.
However the main issue are the other applications that any user would add along with the providers fixed ROM . These have been proven in the netherlands to be a great deal of trouble in every S4 device. I should have stated in my earlier post that the problem was not a common one to all S4 mobiles.
So ...giving you the benefit of the doubt I may have to agree that it isn't all S4 devices that may have the same problem. Regarding to the providers added applications and network drivers it is common here in the netherlands that most S4 devices work erroneous when using other applications likely to Viber or itself.
Note * Viber where tested on the HTC 2.3 up to 3.xx , on every SDRom installation the problems with the application varied. This does not say that its performance on KitKat is obsolete. Therefor the most practical way to learn this is to test your theory and in conclusion share it with the forum. I have choosen DingTone for its easy and light weighted ability to give a crystal clear full duplex connection....but thats a personal choice of my own.
Hope my explanation may of given you new light into a brighter point of view.

Wizzard