I find POP more predictable in the way I use mail across multiple clients, on multiple platforms so any wishlist I might build would have POP compatibility as one of my top 'wishes'. However if I could make Vivaldi work with my email hosts that would be better than nothing.
I suppose for Opera, now they've gone down the 'free' route there is little money to be made with an email client so I understand why the mail client was abandoned. The trouble is, for me, that email clients seem to have become unwieldy behemoths of programs, including multiple options I have no desire for but removing many things I'd like to see, such as a way of manually (and simply) marking emails as read, a unified inbox and an easy way to group an locate emails - particularly unread ones - so I can see which emails I need to respond to.
Opera Mail does almost everything I want, and (mostly) very well. If there was an email client I could buy for macOS that worked as well as Opera Mail, looked as good as Opera Mail and had the feature set of Opera Mail, I'd buy it. Trouble is, most of the 'free' ones require that I subscribe to some online services, have annoying adverts or restrictions of one sort or another. The paid ones don't seem to be any better, they also have subscription requirements, annoying adverts or a massive feature set of functions I don't want without any functions I do want. Hence the reason I'm back using Opera Mail again. I've been using Apple Mail but it has started playing up and keeps (silently) losing my SMTP server settings and then defaults to sending my emails through an email server I only use for incoming emails.