Viber

I wouldn't say there are any good parts of using it. It just monopoly, I was forced to use it for job.

At first, lots of ads between messages, and some even in my push notifications! At second, Viber isn't made for people, half of sticker packs are paid, several more features are also paid. You pay to use the free app, basically. And the most important downside is full dependency on your phone number >:( You can't use it with your email, like other normal messengers. Look, to install Viber on desktop PC, you have to install it and keep it on your phone. Why would I want to use Viber Desktop if I have a phone, that's just idiotic. Yes, I know that mobile phones are more popular that PC nowadays, but really, what if I don't want to use my phone, and mostly spend time on my PC than on my phone? If you delete Viber, and reinstall it, all your messages are Gone! Also, Viber takes a lot of space by downloading and opening all my media in my phone storage. Why don't put a bit of effort, and rent some servers, so we could open photos directly on servers without saving it to our phones, like FB Messenger does, for example?! That's not the full list, but it's my detailed opinion on Rakuten Viber. And well, do you have issues or questions? Rakuten doesn't care, they have No user support.

Bad experience, issues with being "free messenger", filling up phone storage, advertisements everywhere, Strong Dependency on your Phone, and NO user support at all.