In one of my first jobs like 15 years ago at a large software company we had just moved to a monorepo.
It was introduced to counterbalance what many saw as a big mess. Result was a lot of process being introduced which slowed everything down, but that was probably necessary at that stage. To my knowledge the company keeps switching back and forth- but new projects that need to move fast typically are done independently still.
I would expect you need really good training in place to make it work. e.g. Microsoft uses a git monorepo for the Windows codebase; obviously that is not something you could just come in on and do a "git clone" as you might on a small project.
It was introduced to counterbalance what many saw as a big mess. Result was a lot of process being introduced which slowed everything down, but that was probably necessary at that stage. To my knowledge the company keeps switching back and forth- but new projects that need to move fast typically are done independently still.