I have 20 years of email tied up in an SQLite database that, if I dare press "update" on the old Opera, will likely just disappear.
They even released an unintentional Vivaldi preview with the functionality in it... nothing since.
I see no reason to use Vivaldi while it's literally just Chromium in a fancy wrapper. I can get that anywhere... hell, that's basically what Edge will be very soon.
Email clients are 10-a-penny. One that works and imports all my old email (from accounts that no longer physically exist) are not.
There will come a time when I just script the whole damn thing out into Dovecot etc. mailboxes and never touch Opera or Vivaldi ever again, but I was silly enough to believe their years of initial announcements that it was going to come.
They have literally zero USP over just using Chrome.