It’s true Vivaldi is not free open source, the code is owned by Vivaldi however, the source code is freely available to audit which is the main security benefit you would get from a FOSS browser like Brave and Firefox. It is plainly not spyware.

Yes there is the security risk that someone might find an exploit in the source code, and if Vivaldi is notified, users would have to wait for Vivaldi to fix it. As opposed to a third party potentially issuing a patch quicker.

But this is also basically true for Firefox and Brave. If a security flaw is found it’s more than likely going to be the Firefox or Brave team that fixes it first.