The new UI was useful when it was opt-in and we were migrating from on-prem TFS to what was then called VSTS. It made it easy to tell which system you were using.

I'm more used to the old UI but some features are not easily discoverable. I wrote some posts on how to build branches and pull requests as it's not obvious: https://unop.uk/build-and-release-all-pull-request-merge-res...