@TheBalrog have you expanded the "all accounts" view? This is how it looks like for me.
Screenshot from GMail first, taken in the "All Accounts" and showing only unread messages.
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Let's see what we have here, marked with colors:
4x Green: Inbox
4x Yellow: Important (not sure why GMail doesn't have a counter there)
3x blue: "not in INBOX" = Archived (it's a thread of 3 mails showing as one line item)
5x red / hidden in message view: unread spam emails
Now the same thing in Vivaldi Mail ... same counters
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There may be a sync issue involved. You can right-click any folder and choose Advanced - Rerun filter for folder.