I use monorepo approach all the time in my work, it works great for me, the disadvantages is when it comes working with others, a lot of my peers get confused working with it, it also isn’t the best for say granular access compared if you have everything in separate repos.
Most programmers aren't mature enough, or smart enough, to handle a monorepo. It requires too much interaction with code owners, reviewers, and ongoing churn.
Technically google isn't a monorepo, but when I worked there for 7 of the years I worked in google3, which was truly a monorepo, except for Android, and the kernel, and some open source projects... OK, not a monorepo.