I only use Viber with contacts who don't have WhatsApp. The contacts' "Last seen" info is often wrong, which is unhelpful and confusing. (E.g., a friend texted me 10 minutes ago - but according to Viber, he hasn't been online for a week.) Unsolicited messages, stickers and emails = annoying. When I'm fed up and erase the app for good, it notifies everyone in my phone (and I look like a crazy person). Etc. etc. In other words, lots of irritating little issues I've never experienced with WhatsApp.