I think it's strange that there's no sound notifications in messengers selected in the sidebar. (although there's mute sound button in messengers settings). I tested it with Telegram and VKontakte and couldn't hear any.
Also, in Telegram, when you select the chat, it's focuses on the input field, and you can type messages immediately. But in VKontakte you have to focus on the input field manually
Sound notifications are very important, cause there's no point in sidebar messengers if you have to have a messenger page opened to get notifications.
As for the second part - I think all features realized in one supported messenger should be in the others as well, because it's a bit confusing.
English isn't my native language, so I'm apologize if there are some mistakes