My problem with polyrepos is that often organizations end up splitting things too finely, and now I'm unable to make a single commit to introduce a feature because my changes have to live across several repositories. Which makes code review more annoying because you have to tab back and forth to see all the context. It's doubly frustrating when I'm (or my team is) the only people working on those repositories, because now it doesn't feel like it gained any advantages. I know the author addresses this, but I can't imagine projects are typically at the scale they're describing. Certainly it's not my experience.
Also I definitely miss the ability to make changes to fundamental (internal) libraries used by every project. It's too much hassle to track down all the uses of a particular function, so I end up putting that change elsewhere, which means someone else will do it a little different in their corner of the world, which utterly confuses the first person who's unlucky enough to work in both code bases (at the same time, or after moving teams).
Also I definitely miss the ability to make changes to fundamental (internal) libraries used by every project. It's too much hassle to track down all the uses of a particular function, so I end up putting that change elsewhere, which means someone else will do it a little different in their corner of the world, which utterly confuses the first person who's unlucky enough to work in both code bases (at the same time, or after moving teams).