It amazes me that such big projects use monorepos. My company also though about doing so, because we have a few typescript projects that share a common module with business logic and everything lags behind when the common module was upgraded, but I don't know if I should sell them to switch over to a mono repo as well. My biggest pro argument would be the speed up in development and review time, because everything is…
If you follow the design principles outlined in this doc (https://www.lihaoyi.com/post/StrategicScalaStylePrincipleofL...), Scala projects are really refactorable, which is nice.
But I've also worked on lots of large Scala codebases that are completely incomprehensible and painful to change. Scala allows for devs to make beautiful monoliths, but that's not usually what happens.