I have the same observations and I do hope that Viber continues to iron out the bugs.
And just to add to others comments, here are mine.
I can't pivot left and right between conversations - contacts - pubic chats. I have to reach out all the way to the top just to switch the view. I read somewhere that this may be a limitation of the tool that was used to make the app. In any case it's a slight inconvenience.
The pivot issue I guess is the same with the Windows Central app where you can't pivot between the article and comments.
My contacts online status takes awhile to update, it could show "online moments ago" when the user is actually online.
Also, so far I still cannot post a new or update my review in the Store.
My two cents