Like you, I've used Vivaldi for years. I downloaded because of the email client. Opera's M2 was a great mail client. I'm probably in the minority of users who don't use the usual suspects for webmail, and having an integrated mail client is nice. M2, in particular did some nice things:
* Fast full-text search through mail
* No organization: automatic mailing list aggregation; easy contact management; easy email filter by contact or thread
* Keyboard navigable.
* Doesn't default to top-quoting.
* RSS integration.
The M3 client had to be rewritten, one presumes, as a Chrome extension. This had to be pretty awful, considering, and probably explains part of the reason that everything took so long. I've been driving M3 for awhile, and what I really miss is suitable keyboard shortcuts. For the most part I've been very happy with it, though.