Unsurprising
I feel for Vivaldi, and for other smaller browser companies as well. I've often had similar problems with Pale Moon on sites - yet if you spoof the latest Firefox UA, hey-presto the site works and renders fine! Pale Moon currently has a pre-configured "list" of naughty sites for which it'll spoof a Firefox UA by default to work around blocks, which isn't ideal but does mean that most things work correctly whilst reporting the Pale Moon UA to everything else.
The problem here is with over-zealous web developers who have an infuriating habit of testing their site with a few major browsers (which is fair enough), but then blocking everything else (which is stupid). The result is smaller browsers that technically have no problem rendering a site - but are prevented from getting the chance to do so because they aren't one of the "big-few".
Of course the best approach would be for the site to check for required browser features and only pop up a warning message if a feature used by the site is reported as not supported by the user's browser. But of course this approach takes effort - far easier for the lazy developer to just block everything that isn't Google Chrome and then tell everyone to jump on the Google bandwagon...
Biting the hand that feeds IT
