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!"

(https://vivaldi.com/blog/user-agent-changes/)