Basically, you don't want everything be dependant on a central authority - this is a single point of failure. If there's a big security vulnerability in your web browser, but you use a standalone mail client, your mails are most probably safe.
This also adds up to built-in adblocker - who knows if Vivaldi devs will ever go evil and sneak in exceptions in their adblocker? Or if they will sell their web browser, just like their CEO done so with his previous browser, Opera.