Can't mount downloaded .dmg
The only issue I've seen related to this have been people having issues with the Vivaldi dmg itself. This issue is with the MightyText desktop app dmg. If I download with Vivaldi, it can't be mounted (and DiskUtil says it doesn't have a checksum so it can't verify it). The same dmg downloaded with Safari works just fine.
https://mightytext.net/downloads/desktop-beta/mac.html is the link to the dmg.
@EricS I've never had any issues downloading files with Vivaldi. Do you have a "download manager" or any other extensions installed that might be causing problems? If you look at the size of the files that Safari and Vivaldi downloaded, how do they compare?