@circular @Vivaldi
There is always some level of trust using digital services. To lower the risk, you should try understand the service and the company behind. Open Source helps too, however you then trust the people who validates the code. I think Vivaldi has an ok balance of open and closed, as they want to protect some of their front-end code as written in the blog post below.
You will never know for sure. Using internet services, will always include some risk of privacy. It's all about mitigating this risk. What actions do I take to lower the risk, which fits my threatmodel.

https://vivaldi.com/blog/vivaldi-browser-open-source/