So I know that this is one of those topics that's more about preference in 2021. I want to hear from both sides and see if my assumptions need to be updated.

Personally I favor Yarn and have done so for about 4 years working mostly on React projects in teams with 3 - 10 developers. Yarn and it's locked files removed issues of dependencies being a different version and sometimes causing weirdness. Similarly the CI versions have seem to create consistent builds. Finally the lock file is stable and only really changes when a package version is updated, while the last time I used npm the lock file updated a lot more often for no reason.

I would love to hear and have a discussion on the state of NPM and Yarn what the feelings are out there.