In the past Vivaldi used their own user agent string and they ran into a bunch of issues. And they are a chrome derivative! They had to default to the chrome user agent. Here are the examples they cite in their announcement of the decision:
"On Google.com if you present a Vivaldi user agent and arrive via a redirect, the search text box will be misaligned
On Google Docs if you present a Vivaldi user agent you will receive a warning
On Facebook’s WhatsApp web interface if you present a Vivaldi user agent, you cannot enter the site and are advised to switch to one of our competitors
On Microsoft Teams (chat and collaboration website), presenting a Vivaldi user agent will stop you from being able to use the website
On Netflix, presenting a Vivaldi user agent results in a suggestion to install Silverlight to play videos… yes… really… Silverlight!"
When these mega-companies block new competitors it really ought to be seen as collusion. Google, Facebook, and Microsoft certainly have the resources to test and approve the occasional new browser.
They don’t even have the resources to test the most common browsers on every scenario of every page of every application, let alone fix every issue such testing would find.